Sulfonium salt, polymer, polymer making method, resist composition, and patterning process

ABSTRACT

A sulfonium salt having formula (1a) is provided wherein R 1  is H, F, CH 3  or CF 3 , R 1a  to R 1m  are each independently H or a monovalent hydrocarbon group, L is a single bond or divalent hydrocarbon group, X is a divalent alkylene group optionally substituted with fluorine, and n is 0 or 1. The sulfonium salt having a polymerizable anion provides for efficient scission of acid labile groups in a chemically amplified resist composition, and it is a very useful monomer from which a base resin for resist use is prepared.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2012-269446 filed in Japan on Dec. 10, 2012,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to (1) a sulfonium salt having a polymerizableanion useful as a source for photoacid generator and sulfonic acidpolymer, (2) a polymer comprising recurring units derived from thesulfonium salt, capable of generating a sulfonic acid in response tohigh-energy radiation or heat, (3) a method of preparing the polymer,(4) a resist composition comprising the polymer, and (5) a patterningprocess using the resist composition.

BACKGROUND ART

While a number of recent efforts are being made to achieve a finerpattern rule in the drive for higher integration and operating speeds inLSI devices, DUV and EUV lithography processes are thought to holdparticular promise as the next generation in microfabricationtechnology. In particular, photolithography using an ArF excimer laseras the light source is requisite to the micropatterning techniquecapable of achieving a feature size of 0.13 μm or less.

The ArF lithography started partial use from the fabrication of 130-nmnode devices and became the main lithography since 90-nm node devices.For the next 45-nm node devices which required an advancement to reducethe wavelength of exposure light, the F₂ lithography of 157 nmwavelength became a candidate. However, for the reasons that theprojection lens uses a large amount of expensive CaF₂ single crystal,the scanner thus becomes expensive, hard pellicles are introduced due tothe extremely low durability of soft pellicles, the optical system mustbe accordingly altered, and the etch resistance of resist is low; the F₂lithography was postponed and instead, the early introduction of ArFimmersion lithography was advocated. This enables mass-scale productionof 45-nm node devices. For the mass-scale production of 32-nm nodedevices, the double patterning process utilizing sidewall spacertechnology is used although the process suffers from complexity andlength.

For the fabrication of 32-nm node and subsequent devices, the EUVlithography using an exposure wavelength of 13.5 nm which is shorterthan the conventional lasers by one order of magnitude and thusfeaturing improved resolution is expected rather than the doublepatterning process with noticeable costs. Efforts are focused on the EUVlithography.

In the EUV lithography, a low laser power and light attenuation byreflecting mirror lead to a reduced quantity of light. Then light with alow intensity reaches the wafer surface. It is urgently demanded todevelop a high-sensitivity resist material in order to gain a throughputdespite a low light quantity. However, a trade-off relationship ofsensitivity is pointed out that the sensitivity of resist material canbe increased at the sacrifice of resolution and edge roughness (LER,LWR).

As the circuit line width is reduced by the recent rapid advance oftechnology, the degradation of contrast by acid diffusion becomes moreserious for the resist material. This is because the pattern featuresize is approaching the diffusion length of acid. Acid diffusion leadsto degradations of mask fidelity and pattern rectangularity andnon-uniformity of a fine line pattern, i.e., line width roughness (LWR).Accordingly, to gain more benefits from a reduction of exposure lightwavelength and an increase of lens NA, an increase in dissolutioncontrast and suppression of acid diffusion are required more than in theprior art resist materials.

One approach to overcome these problems is to bind a PAG in a polymer.For instance, aiming to improve sensitivity, Patent Document 1 proposesa polymer using an acryloyloxyphenyldiphenylsulfonium salt as a monomer.Patent Document 2 proposes to incorporate the monomer into apolyhydroxystyrene resin for improving the LWR of this base resin.However, since the sulfonium salt is bound at its cation side to thepolymer, the sulfonic acid generated therefrom upon exposure tohigh-energy radiation is equivalent to the sulfonic acids generated byconventional PAGs. These proposals are thus unsatisfactory to overcomethe outstanding problems. Also, aiming to improve sensitivity and resistpattern profile, Patent Document 3 discloses sulfonium salts having ananion side incorporated into the polymer backbone such aspolystyrenesulfonic acid. The acids generated therefrom arearenesulfonic and alkylsulfonic acid derivatives which have too low anacid strength to sever acid labile groups, especially acid labile groupsin acrylate-derived base resins. The acrylate resins are commonly usednot only in the ArF chemically amplified lithography offering a finefeature size, but also in the EB and EUV lithography processes. Also avariety of anion-bound resins capable of generating an acid having highacid strength have been developed. Patent Document 4 discloses a polymerhaving a difluoroethanesulfonic acid anion in the backbone. PatentDocuments 5 and 6 disclose a polymerizable sulfonium salt having apartially fluorinated sulfonic acid anion and a resin obtainedtherefrom. Acid diffusion is suppressed by incorporating a strongacid-generating anion in the backbone of a base resin. Although someimprovements are made in resist properties including mask fidelity,pattern rectangularity and LWR, they are still unsatisfactory.

In the EUV laser source of the laser-produced plasma (LPP) methodwherein CO₂ laser light is irradiated to tin particles to emit EUV ofwavelength 13.5 nm, weak light of longer wavelength 140 to 300 nm isemitted besides the desired EUV. This longer wavelength like is known asout-of-band (OOB) light. Since OOB floods over the entire surface, theresist exposed to OOB is reduced in contrast and experiences a filmthickness loss in the otherwise unexposed region. The EUV microstepperis loaded with a Zr filter as a means for cutting off OOB light, but thequantity of light is reduced thereby. The EUV scanner may not be loadedwith the filter because a reduction of light quantity is not permissiblefor the goal of enhancing the throughput. In the EUV lithography, thereis a need for a resist material which is highly sensitive to EUV, butless sensitive to OOB.

For the development of such resist materials, the cation structure ofsulfonium salt PAG is important. Patent Document 7 (JP-A 2011-138107,paragraph [0052]) describes a polymer-bound acid generator having a highsensitivity to EUV light, but a low sensitivity to OOB light. Since itslithographic characteristics are still unsatisfactory, it is desired tohave a resist material having a lower OOB sensitivity. Non-PatentDocument 1 describes the superiority of a protective film which isformed on top of the resist layer for cutting off OOB light.

CITATION LIST

-   -   Patent Document 1: JP-A H04-230645    -   Patent Document 2: JP-A 2005-084365    -   Patent Document 3: JP 3613491 (U.S. Pat. No. 5,945,250)    -   Patent Document 4: JP-A 2007-197718 (U.S. Pat. No. 7,932,334)    -   Patent Document 5: WO 08/056795    -   Patent Document 6: JP-A 2008-133448 (U.S. Pat. No. 7,569,326)    -   Patent Document 7: JP-A 2011-138107    -   Non-Patent Document 1: Proc. SPIE Vol. 7969, p 796916-1 (2011)

DISCLOSURE OF INVENTION

An object of the invention is to provide (1) an onium salt, (2) apolymer comprising recurring units derived from the onium salt, (3) aresist composition comprising the polymer, and (4) a patterning processusing the resist composition, wherein the resist composition exhibits ahigh resolution and forms a pattern of good profile and reduced LWR whenprocessed by photolithography using high-energy radiation such as ArFexcimer laser, EUV or EB. A further object is to provide a PAG and aresist composition comprising the PAG, for use in the EUV lithography,which have a high sensitivity to EUV, but no or low sensitivity to OOB.

The inventors have found that a resist composition using a polymercomprising recurring units of a sulfonium salt having the generalformula (1) shown below exhibits improved characteristics includingresolution and LWR when processed by photolithography, especially EUVlithography. The sulfonium salt exhibits low OOB sensitivity whenprocessed by EUV lithography, and is effective for improving the profileof a resist pattern.

In a first aspect, the invention provides a sulfonium salt having thegeneral formula (1a):

wherein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, R^(1a) toR^(1m) are each independently hydrogen or a C₁-C₂₀ straight, branched orcyclic, monovalent hydrocarbon group which may be substituted with orseparated by a heteroatom, L is a single bond or a C₁-C₂₀ straight,branched or cyclic, divalent hydrocarbon group which may be substitutedwith or separated by a heteroatom, X is a C₁-C₅ divalent alkylene groupin which some or all hydrogen atoms may be substituted by fluorineatoms, and n is 0 or 1.

One preferred embodiment is a sulfonium salt having the general formula(1b):

wherein R¹, R^(1a) to R^(1m) are as defined above, L¹ is a single bondor a C₁-C₂₀ straight, branched or cyclic, divalent hydrocarbon groupwhich may be substituted with or separated by a heteroatom, A ishydrogen or trifluoromethyl, and n is 0 or 1, with the proviso that n is0 when L¹ is a single bond.

In a second aspect, the invention provides a polymer comprisingrecurring units having the general formula (2a):

wherein R¹, R^(1a) to R^(1m), L, X and n are as defined above.

One preferred embodiment is a polymer comprising recurring units havingthe general formula (2b):

wherein R¹, R^(1a) to R^(1m), L¹, A, and n are as defined above.

In a preferred embodiment, the polymer may further comprise recurringunits having the general formula (3) and/or (4):

wherein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, R² ishalogen or a C₁-C₁₀ alkyl group, L′ is a single bond or a C₁-C₁₀divalent organic group which may be substituted with oxygen, p is aninteger of 0 to 3, q is 1 or 2, N is an integer of 0 to 2, Z is a singlebond, phenylene, naphthylene or (backbone)-C(═O)—O—Z′—, Z′ is a C₁-C₁₀straight, branched or cyclic alkylene group which may have a hydroxylradical, ether bond, ester bond or lactone ring, or phenylene ornaphthylene group, and XA is an acid labile group.

In a further preferred embodiment, the polymer may further compriserecurring units having the general formula (5):

wherein R¹ is as defined above, and YL is hydrogen or a polar grouphaving at least one structure selected from the group consisting ofhydroxyl, cyano, carbonyl, carboxyl, ether bond, ester bond, sulfonicacid ester bond, carbonate bond, lactone ring, sultone ring andcarboxylic anhydride.

In a third aspect, the invention provides a resist compositioncomprising the polymer defined above as a base resin.

Another embodiment is a resist composition comprising the polymerdefined above and a polymer free of recurring units having formulae (2a)and (2b) as a base resin.

Typically the resist composition is a chemically amplified resistcomposition. In this regard, the resist composition may further comprisea basic compound and an organic solvent, and optionally, a non-polymericacid generator and a surfactant which is insoluble in water and solublein alkaline developer.

In a fourth aspect, the invention provides a pattern forming processcomprising the steps of applying the resist composition defined aboveonto a substrate to form a coating, baking, exposing the coating tohigh-energy radiation, and developing the exposed coating in adeveloper.

Preferably, the exposure step is carried out by immersion lithographyusing a liquid having a refractive index of at least 1.0 between theresist coating and a projection lens.

In a preferred embodiment, a protective film is coated on the resistcoating prior to the exposure step. In this case, immersion lithographyis carried out while the liquid is held between the protective film andthe projection lens.

Typically, the high-energy radiation is electron beam or soft X-rayhaving a wavelength of 3 to 15 nm.

In a fifth aspect, the invention provides a method for preparing thepolymer of the second aspect, comprising the step of reacting a polymercomprising recurring units having the general formula (6a) with asulfonium salt having the general formula (7).

Herein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, R^(1a) toR^(1m) are each independently hydrogen or a C₁-C₂₀ straight, branched orcyclic, monovalent hydrocarbon group which may be substituted with orseparated by a heteroatom, L is a single bond or a C₁-C₂₀ straight,branched or cyclic, divalent hydrocarbon group which may be substitutedwith or separated by a heteroatom, X is a C₁-C₅ divalent alkylene groupin which some or all hydrogen atoms may be substituted by fluorineatoms, n is 0 or 1, Za⁺ is a lithium ion, sodium ion, potassium ion, orammonium cation of the general formula (8):

(R³)₄N⁺  (8)

wherein R³ is each independently hydrogen, a substituted orunsubstituted, C₁-C₁₀ straight, branched or cyclic alkyl, alkenyl oroxoalkyl group, or substituted or unsubstituted C₆-C₁₈ aryl, aralkyl oraryloxoalkyl group, or any two or more of R³ may bond together to form aring with N, and Xa is an anion.

A method for preparing the polymer of the preferred embodiment of thesecond aspect is also provided, the method comprising the step ofreacting a polymer comprising recurring units having the general formula(6b) with a sulfonium salt having the general formula (7).

Herein R¹, R^(1a) to R^(1m), L¹, A, n, Za⁺, and Xa⁻ are as definedabove.

Advantageous Effects of Invention

Since the sulfonium salt having a polymerizable anion provides forefficient scission of acid labile groups in a chemically amplifiedresist composition, it is very useful as a monomer from which a baseresin in a photosensitive resist composition is prepared. Aphotosensitive resist composition using the polymer as base resin isimproved in resolution and LWR and thus best suited for precisemicropatterning by photolithography, especially EUV lithography. Sincethe resist composition exhibits low OOB sensitivity and high EUVsensitivity when processed by EUV lithography, a resist pattern isformed at a high contrast due to a reduced film thickness loss in theunexposed region.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing ¹H-NMR spectroscopy of PAG-2 in SynthesisExample 1-7.

FIG. 2 is a diagram showing ¹⁹F-NMR spectroscopy of PAG-2 in SynthesisExample 1-7.

FIG. 3 is a diagram showing ¹H-NMR spectroscopy of PAG-3 in SynthesisExample 1-8.

FIG. 4 is a diagram showing ¹⁹F-NMR spectroscopy of PAG-3 in SynthesisExample 1-8.

DESCRIPTION OF EMBODIMENTS

The singular forms “a,” “an” and “the” include plural referents unlessthe context clearly dictates otherwise. “Optional” or “optionally” meansthat the subsequently described event or circumstances may or may notoccur, and that description includes instances where the event orcircumstance occurs and instances where it does not. The notation(Cn-Cm) means a group containing from n to m carbon atoms per group.

The abbreviations have the following meaning.

EB: electron beamUV: ultravioletEUV: extreme ultravioletPAG: photoacid generatorPEB: post-exposure bakeLWR: line width roughness

The term “high-energy radiation” is intended to encompass UV, deep UV,EUV, EB, x-ray, excimer laser, gamma-ray and synchrotron radiation.

One embodiment of the invention is a sulfonium salt having apolymerizable anion, represented by the general formula (1a).

In formula (1a), R¹ is hydrogen, fluorine, methyl or trifluoromethyl.

R^(1a) to R^(1m) are each independently hydrogen or a C₁-C₂₀ straight,branched or cyclic, monovalent hydrocarbon group which may besubstituted with or separated by a heteroatom. Suitable groups of R^(1a)to R^(1m) include hydrogen, methyl, ethyl, propyl, isopropyl, n-butyl,sec-butyl, tert-butyl, tert-amyl, n-pentyl, n-hexyl, n-octyl, n-nonyl,n-decyl, cyclopentyl, cyclohexyl, 2-ethylhexyl, cyclopentylmethyl,cyclopentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclohexylethyl,cyclohexylbutyl, norbornyl, oxanorbornyl,tricyclo[5.2.1.0^(2,6)]decanyl, and adamantyl. Also included aresubstituted forms of the foregoing groups in which one or more hydrogenatoms are substituted by a heteroatom or atoms such as oxygen, sulfur,nitrogen, and halogen or which may be separated by a heteroatom such asoxygen, sulfur or nitrogen. As a result of substitution or separation, ahydroxyl, cyano, carbonyl, ether bond, ester bond, sulfonic acid esterbond, carbonate bond, lactone ring, sultone ring, carboxylic anhydrideor haloalkyl group may form or intervene. Inter alia, hydrogen, methyl,methoxy, tert-butyl and tert-butoxy are preferred. More preferably,R^(1a) is hydrogen, methyl, methoxy, tert-butyl or tert-butoxy, andR^(1b) to R^(1m) are hydrogen.

L is a single bond or a C₁-C₂₀ straight, branched or cyclic, divalenthydrocarbon group which may be substituted with or separated by aheteroatom. Suitable divalent hydrocarbon groups include straightalkanediyl groups such as methylene, ethylene, propane-1,3-diyl,butane-1,4-diyl, pentane-1,5-diyl, hexane-1,6-diyl, heptane-1,7-diyl,octane-1,8-diyl, nonane-1,9-diyl, decane-1,10-diyl, undecane-1,11-diyl,dodecane-1,12-diyl, tridecane-1,13-diyl, tetradecane-1,14-diyl,pentadecane-1,15-diyl, hexadecane-1,16-diyl, and heptadecane-1,17-diyl;branched alkanediyl groups obtained by adding a side chain such asmethyl, ethyl, propyl, isopropyl, butyl, sec-butyl or tert-butyl to theforegoing straight alkanediyl groups; saturated cyclic hydrocarbongroups such as cyclopentanediyl, cyclohexanediyl, norbornanediyl, andadamantanediyl; and unsaturated cyclic hydrocarbon groups such asphenylene and naphthylene. L may also be a combination of two or more ofthe foregoing groups. Also included are substituted forms of theforegoing groups in which one or more hydrogen atoms are substituted bya heteroatom or atoms such as oxygen, sulfur, nitrogen, and halogen. Asa result of substitution, a hydroxyl, cyano, carbonyl, ether bond, esterbond, sulfonic acid ester bond, carbonate bond, lactone ring, sultonering, carboxylic anhydride or haloalkyl group may form.

X is a C₁-C₅ divalent alkylene group in which some or all hydrogen atomsmay be substituted by fluorine atoms. Suitable alkylene groups includemethylene, ethylene, propane-1,3-diyl, butane-1,4-diyl,pentane-1,5-diyl, difluoromethylene, tetrafluoroethylene,1,1,2-trifluoroethylene, hexafluoropropane-1,3-diyl, andoctafluorobutane-1,4-diyl. In particular, those of formula (1a) whereinthe hydrogen of methylene group at alpha-position of sulfonic acid issubstituted by fluorine are preferred.

The subscript n is equal to 0 or 1.

The preferred sulfonium salt has the general formula (1b).

In formula (1b), R¹, R^(1a) to R^(1m), and n are as defined above. L¹ isa single bond or a C₁-C₂₀ straight, branched or cyclic, divalenthydrocarbon group which may be substituted with or separated by aheteroatom, with the proviso that n is 0 when L¹ is a single bond.Exemplary groups of L¹ are the same as illustrated for L in formula(1a). A is hydrogen or trifluoromethyl, preferably trifluoromethyl.

Exemplary structures of the sulfonium salt are shown below, but notlimited thereto.

In these formulae, R¹ and A are as defined above.

It is now described how to synthesize the polymerizable anion-containingsulfonium salt having formula (1a). The desired sulfonium salt isobtainable from ion exchange reaction between a salt (e.g., lithium,sodium, potassium or ammonium salt) of a sulfonic acid having apolymerizable functional group (e.g., (meth)acryloyl or vinyl) and asulfonium salt compound having the structure of the cation moiety informula (1a). For the ion exchange reaction, reference may be made toJP-A 2007-145797, for example.

For the synthesis of a sulfonium salt compound having the structure ofthe cation moiety in formula (1a), the well-known method for thesynthesis of sulfonium salts is applicable. For example, the sulfoniumsalt compound may be synthesized by reacting thioxanthone or itsderivative with a diaryl iodonium salt in the presence of a coppercatalyst such as copper benzoate.

It is then described how to synthesize the sulfonium salt having formula(1b) as the preferred embodiment of the invention. First, the synthesisof a sulfonium salt having formula (1b) wherein A is hydrogen isillustrated.

A sulfonium salt having 1,1-difluoro-2-hydroxyethane sulfonate isprepared. To this end, 2-bromo-2,2-diluoroethanol is reacted withcarboxylic acid chloride to form 2-bromo-2,2-difluoroethylalkanecarboxylate or 2-bromo-2,2-difluoroethylarene carboxylate, which is inturn reacted with a sulfur compound such as sodium dithionite forconverting bromo group to sodium sulfinate, which is then converted tosodium sulfonate using an oxidizing agent such as hydrogen peroxide.

Herein R⁹ is a C₁-C₂₀ straight, branched or cyclic alkyl groupoptionally containing a heteroatom.

The steps of esterification, conversion of alkane halide to sodiumsulfinate, and conversion to sulfonic acid are well known, as discussedin detail in JP-A 2004-002252, for example.

The resulting sodium sulfonate is subjected to ion exchange reactionwith a sulfonium salt compound, yielding the desired sulfonium salt.

Herein R⁹ and R^(1a) to R^(1m) are as defined above. X⁻ is a counteranion, examples of which include, but are not limited to, halide ionssuch as I⁻, Br⁻ and Cl⁻; sulfonic acid or alkylsulfonic acid anions suchas sulfate anion and methylsulfate anion; carboxylic acid anions such asacetate and benzoate; alkanesulfonate ions such as methanesulfonate andpropanesulfonate; arenesulfonate ions such as benzenesulfonate andp-toluenesulfonate; and hydroxide.

Further, a sulfonium salt having 1,1-difluoro-2-hydroxyethanesulfonatecan be synthesized by subjecting the acyl group: R⁹CO— introduced asabove to ester hydrolysis or solvolysis. This step is outlined below.

Herein R⁹ and R^(1a) to R^(1m) are as defined above, and Me stands formethyl.

The sulfonium salt having 1,1-difluoro-2-hydroxyethane-sulfonate,synthesized as above, is reacted with a corresponding carboxylic halideunder basic conditions, yielding the desired sulfonium salt of formula(1b) wherein A is hydrogen.

By a choice of a suitable carboxylic halide at this point, L¹ in formula(1b) may be changed. Namely, even when the composition of a base resinis altered, the sulfonium salt of the invention can be easily redesignedto an optimum salt for a particular base resin. The sulfonium salt findsa wide range of application.

Next, the synthesis of a sulfonium salt having formula (1b) wherein A istrifluoromethyl is illustrated.

A sulfonium salt having 1,1,3,3,3-pentafluoro-2-hydroxypropanesulfonateis synthesized instead of the sulfonium salt having1,1-difluoro-2-hydroxyethanesulfonate. This is followed by the sameprocedures as in the above embodiment wherein A is hydrogen, yieldingthe desired sulfonium salt of formula (1b) wherein A is trifluoromethyl.In the embodiment wherein A is trifluoromethyl, L¹ in formula (1b) maybe changed likewise.

With respect to the synthesis of the sulfonium salt having1,1,3,3,3-pentafluoro-2-hydroxypropanesulfonate, reference may be madeto JP-A 2007-145804.

While several methods for the synthesis of a sulfonium salt havingformula (1b) have been described, they are merely exemplary and notintended to limit the invention thereto.

Another embodiment of the invention is a polymer orhigh-molecular-weight compound capable of generating a sulfonic acid inresponse to high-energy radiation or heat. The polymer is characterizedby comprising recurring units having the general formula (2a).

Herein R¹, R^(1a) to R^(1m), L, X, and n are as defined above.

It is noted that Patent Document 7 (JP-A 2011-138107, paragraph [0052])describes a polymer-bound acid generator having a high sensitivity toEUV light, but a low sensitivity to OOB light. Patent Document 6 (JP-A2008-133448, paragraph [0022]) describes a sulfonium salt having a ringstructure, whose sensitivity to KrF and ArF exposure is low and whosesensitivity to EUV exposure is substantially equal to that oftriphenylsulfonium salts. Namely, the sulfonium salt having a ringstructure described in JP-A 2008-133448, paragraph [0022] has highresistance to OOB light and satisfactory sensitivity to EUV exposure andthus finds advantageous use especially in the EUV lithography. Ascompared with the sulfonium salt having a ring structure described inJP-A 2008-133448, paragraph [0022], the sulfonium salt of the inventionhas low sensitivity to KrF and ArF exposure and high sensitivity to EUVexposure, leading to satisfactory resist performance. For example, aresist pattern of higher contrast and better profile can be formed. Itis believed that the use of a sulfonium salt having lower sensitivity toOOB contributes to improvements in resist performance. Additionally,since the base resin used herein has the specific sulfonium saltincorporated in polymer units, acid diffusion is suppressed and acid isuniformly dispersed. These contribute to improvements in such parametersas LWR and exposure latitude.

More preferably, the polymer comprises recurring units having thegeneral formula (2b).

Herein R¹, R^(1a) to R^(1m), L¹, A, and n are as defined above.

The structure of formula (2b) is very advantageous in that an optimummolecular design for a particular application is possible since L¹ canbe changed among a variety of groups as previously alluded to. It has alow degree of fluorine substitution and an ester structure, which ensurefurther advantages including decomposition and a minimized environmentalload.

In addition to the recurring units having formula (2a) or (2b), thepolymer may further comprise recurring units having the general formula(3) and/or (4).

Herein R¹ is hydrogen, fluorine, methyl or trifluoromethyl. R² ishalogen or a C₁-C₁₀ alkyl group. L′ is a single bond or a C₁-C₁₀divalent organic group which may be substituted with oxygen. Thesubscript p is an integer of 0 to 3, q is 1 or 2, N is an integer of 0to 2. Z is a single bond, phenylene, naphthylene or(backbone)-C(═O)—O—Z′—, wherein Z′ is a C₁-C₁₀ straight, branched orcyclic alkylene group which may have a hydroxyl radical, ether bond,ester bond or lactone ring, or a phenylene or naphthylene group. XA isan acid labile group.

Specifically, L′ is a single bond or a C₁-C₁₀ divalent organic groupwhich may be substituted with oxygen. Suitable organic groups includemethylene, ethylene, propane-1,3-diyl, butane-1,4-diyl,pentane-1,5-diyl, and hexane-1,6-diyl. Preferably L′ is a single bond,methylene or ethylene. Some hydrogen atoms in such group may besubstituted by an oxygen atom, or an oxygen atom may intervene in thegroup. As a result, an ether or ester bond may be formed or interposedin the group. N is an integer of 0 to 2, preferably 0 or 1. R² ishalogen or a C₁-C₁₀ alkyl group, preferably R² being fluorine or methyl.The subscript p is an integer of 0 to 3, preferably 0, and q is 1 or 2,preferably 1.

Exemplary structures of the unit having formula (3) are shown below.

Since the phenolic hydroxyl-containing recurring units of formula (3)are presumed effective for swell suppression and acid generationefficiency in EB and EUV lithography, owing to their hydroxyl group andaromatic ring structure, they are expected to contribute to improvementsin LWR and sensitivity. Accordingly, a resist composition comprising theinventive polymer is useful in EB and EUV lithography.

Examples of the structure having formula (4) wherein Z is a variant areshown below.

Herein R¹ is as defined above, and XA is an acid labile group.

Under the action of acid, a polymer comprising recurring units offormula (4) is decomposed to generate carboxylic acid, turning to be analkali soluble polymer. The acid labile group represented by XA may beselected from a variety of such groups. Examples of the acid labilegroup include groups of the following general formulae (L1) to (L4),tertiary alkyl groups of 4 to 20 carbon atoms, preferably 4 to 15 carbonatoms, trialkylsilyl groups in which each alkyl moiety has 1 to 6 carbonatoms, and oxoalkyl groups of 4 to 20 carbon atoms.

In these formulae, the broken line denotes a valence bond.

In formula (L1), R^(L01) and R^(L02) each are hydrogen or a straight,branched or cyclic alkyl group of 1 to 18 carbon atoms, preferably 1 to10 carbon atoms. Exemplary alkyl groups include methyl, ethyl, propyl,isopropyl, n-butyl, sec-butyl, tert-butyl, cyclopentyl, cyclohexyl,2-ethylhexyl, n-octyl, norbornyl, tricyclodecanyl, tetracyclododecanyl,and adamantyl. R^(L03) is a monovalent hydrocarbon group of 1 to 18carbon atoms, preferably 1 to 10 carbon atoms, which may contain ahetero atom such as oxygen, examples of which include unsubstitutedstraight, branched or cyclic alkyl groups and substituted forms of suchalkyl groups in which some hydrogen atoms are replaced by hydroxyl,alkoxy, oxo, amino, alkylamino or the like, or in which an oxygen atomintervenes between carbon atoms. Exemplary straight, branched or cyclicalkyl groups include methyl, ethyl, propyl, isopropyl, n-butyl,sec-butyl, tert-butyl, cyclopentyl, cyclohexyl, 2-ethylhexyl, n-octyl,norbornyl, tricyclodecanyl, tetracyclododecanyl, and adamantyl.Illustrative examples of the substituted alkyl groups are shown below.

A pair of R^(L01) and R^(L02), R^(L01) and R^(L03), or R^(L02) andR^(L03) may bond together to form a ring with the carbon and oxygenatoms to which they are attached. Each of R^(L01), R^(L02) and R^(L03)is a straight or branched alkylene group of 1 to 18 carbon atoms,preferably 1 to 10 carbon atoms when they form a ring.

In formula (L2), R^(L04) is a tertiary alkyl group of 4 to 20 carbonatoms, preferably 4 to 15 carbon atoms, a trialkylsilyl group in whicheach alkyl moiety has 1 to 6 carbon atoms, an oxoalkyl group of 4 to 20carbon atoms, or a group of formula (L1). Exemplary tertiary alkylgroups are tert-butyl, tert-amyl, 1,1-diethylpropyl,2-cyclopentylpropan-2-yl, 2-cyclohexylpropan-2-yl,2-(bicyclo[2.2.1]heptan-2-yl)propan-2-yl, 2-(adamantan-1-yl)propan-2-yl,1-ethylcyclopentyl, 1-butylcyclopentyl, 1-ethylcyclohexyl,1-butylcyclohexyl, 1-ethyl-2-cyclopentenyl, 1-ethyl-2-cyclohexenyl,2-methyl-2-adamantyl, and 2-ethyl-2-adamantyl. Exemplary trialkylsilylgroups are trimethylsilyl, triethylsilyl, and dimethyl-tert-butylsilyl.Exemplary oxoalkyl groups are 3-oxocyclohexyl, 4-methyl-2-oxooxan-4-yl,and 5-methyl-2-oxooxolan-5-yl. Letter y is an integer of 0 to 6.

In formula (L3), R^(L05) is a substituted or unsubstituted, straight,branched or cyclic C₁-C₈ alkyl group or a substituted or unsubstitutedC₆-C₂₀ aryl group. Examples of the optionally substituted alkyl groupinclude straight, branched or cyclic alkyl groups such as methyl, ethyl,propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, tert-amyl, n-pentyl,n-hexyl, cyclopentyl, and cyclohexyl, and substituted forms of suchgroups in which some hydrogen atoms are substituted by hydroxyl, alkoxy,carboxyl, alkoxycarbonyl, oxo, amino, alkylamino, cyano, mercapto,alkylthio, sulfo or other groups. Examples of the optionally substitutedaryl groups include phenyl, methylphenyl, naphthyl, anthryl,phenanthryl, and pyrenyl. Letter m′ is equal to 0 or 1, n′ is equal to0, 1, 2 or 3, and 2m′+n′ is equal to 2 or 3.

In formula (L4), R^(L06) is a substituted or unsubstituted, straight,branched or cyclic C₁-C₈ alkyl group or a substituted or unsubstitutedC₆-C₂₀ aryl group. Examples of these groups are the same as exemplifiedfor R^(L05). R^(L07) to R^(L16) independently represent hydrogen ormonovalent C₁-C₁₅ hydrocarbon groups. Exemplary hydrocarbon groups arestraight, branched or cyclic alkyl groups such as methyl, ethyl, propyl,isopropyl, n-butyl, sec-butyl, tert-butyl, tert-amyl, n-pentyl, n-hexyl,n-octyl, n-nonyl, n-decyl, cyclopentyl, cyclohexyl, cyclopentylmethyl,cyclopentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclohexylethyland cyclohexylbutyl, and substituted forms of these groups in which somehydrogen atoms are replaced by hydroxyl, alkoxy, carboxyl,alkoxycarbonyl, oxo, amino, alkylamino, cyano, mercapto, alkylthio,sulfo or other groups. Alternatively, two of R^(L07) to R^(L16), takentogether, form a ring with the carbon atom to which they are attached(for example, a pair of R^(L07) and R^(L08), R^(L07) and R^(L09),R^(L07) and R^(L10), R^(L08) and R^(L10), R^(L09) and R^(L10), R^(L11)and R^(L12), or R^(L13) and R¹⁴ form a ring). Each of R^(L07) to R^(L16)represents a divalent C₁-C₁₅ hydrocarbon group when they form a ring,examples of which are the ones exemplified above for the monovalenthydrocarbon groups, with one hydrogen atom being eliminated. Two ofR^(L07) to R^(L16) which are attached to vicinal carbon atoms may bondtogether directly to form a double bond (for example, a pair of R^(L07)and R^(L09), R^(L09) and R^(L15), R^(L13) and R^(L15), or R^(L14) andR^(L15)).

Of the acid labile groups of formula (L1), the straight and branchedones are exemplified by the following groups.

Of the acid labile groups of formula (L1), the cyclic ones are, forexample, tetrahydrofuran-2-yl, 2-methyltetrahydrofuran-2-yl,tetrahydropyran-2-yl, and 2-methyltetrahydropyran-2-yl.

Examples of the acid labile groups of formula (L2) includetert-butoxycarbonyl, tert-butoxycarbonylmethyl, tert-amyloxycarbonyl,tert-amyloxycarbonylmethyl, 1,1-diethylpropyloxycarbonyl,1,1-diethylpropyloxycarbonylmethyl, 1-ethyl cyclopentyloxycarbonyl,1-ethylcyclopentyloxycarbonylmethyl, 1-ethyl-2-cyclopentenyloxycarbonyl,1-ethyl-2-cyclopentenyloxycarbonylmethyl, 1-ethoxyethoxycarbonylmethyl,2-tetrahydropyranyloxycarbonylmethyl, and2-tetrahydrofuranyloxycarbonylmethyl groups.

Examples of the acid labile groups of formula (L3) include1-methylcyclopentyl, 1-ethylcyclopentyl, 1-n-propylcyclopentyl,1-isopropylcyclopentyl, 1-n-butylcyclopentyl, 1-sec-butylcyclopentyl,1-cyclohexylcyclopentyl, 1-(4-methoxy-n-butyl)cyclopentyl,1-methylcyclohexyl, 1-ethylcyclohexyl, 3-methyl-1-cyclopenten-3-yl,3-ethyl-1-cyclopenten-3-yl, 3-methyl-1-cyclohexen-3-yl, and3-ethyl-1-cyclohexen-3-yl groups.

Of the acid labile groups having formula (L4), groups having thefollowing formulas (L4-1) to (L4-4) are preferred.

In formulas (L4-1) to (L4-4), the broken line denotes a bonding site anddirection. R^(L41) is each independently a monovalent hydrocarbon group,typically a straight, branched or cyclic C₁-C₁₀ alkyl group, such asmethyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl,tert-amyl, n-pentyl, n-hexyl, cyclopentyl and cyclohexyl.

For formulas (L4-1) to (L4-4), there can exist enantiomers anddiastereomers. Each of formulae (L4-1) to (L4-4) collectively representsall such stereoisomers. Such stereoisomers may be used alone or inadmixture.

For example, the general formula (L4-3) represents one or a mixture oftwo selected from groups having the following general formulas (L4-3-1)and (L4-3-2).

Similarly, the general formula (L4-4) represents one or a mixture of twoor more selected from groups having the following general formulas(L4-4-1) to (L4-4-4).

Each of formulas (L4-1) to (L4-4), (L4-3-1) and (L4-3-2), and (L4-4-1)to (L4-4-4) collectively represents an enantiomer thereof and a mixtureof enantiomers.

It is noted that in the above formulas (L4-1) to (L4-4), (L4-3-1) and(L4-3-2), and (L4-4-1) to (L4-4-4), the bond direction is on the exoside relative to the bicyclo[2.2.1]heptane ring, which ensures highreactivity for acid catalyzed elimination reaction (see JP-A2000-336121). In preparing these monomers having a tertiary exo-alkylgroup of bicyclo[2.2.1]heptane skeleton as a substituent group, theremay be contained monomers substituted with an endo-alkyl group asrepresented by the following formulas (L4-1-endo) to (L4-4-endo). Forgood reactivity, an exo proportion of at least 50 mol % is preferred,with an exo proportion of at least 80 mol % being more preferred.

Illustrative examples of the acid labile group of formula (L4) are givenbelow.

Examples of the tertiary C₄-C₂₀ alkyl groups, trialkyl-silyl groups inwhich each alkyl moiety has 1 to 6 carbon atoms, and C₄-C₂₀ oxoalkylgroups are as exemplified for R^(L04).

Illustrative examples of the recurring units of formula (4) are givenbelow, but not limited thereto.

While the foregoing examples correspond to those units wherein Z is asingle bond, Z which is other than a single bond may be combined withsimilar acid labile groups. Examples of units wherein Z is other than asingle bond are substantially the same as illustrated above.

The polymer may further comprise additional units, typically recurringunits having the general formula (5).

Herein R¹ is hydrogen, fluorine, methyl or trifluoromethyl. YL ishydrogen or a polar group having one or more structures selected fromthe group consisting of hydroxyl, cyano, carbonyl, carboxyl, ether bond,ester bond, sulfonic acid ester link, carbonate, lactone ring, sultonering, and carboxylic anhydride.

Illustrative, non-limiting examples of the recurring units havingformula (5) are shown below.

Of the recurring units having formula (5), if used, those units having alactone ring as the polar group are most preferred.

On use, the recurring units having formula (5) are copolymerized withthe recurring units having formula (2a), (3) and the optional recurringunits having formulae (3) and (4), although they may be furthercopolymerized with other recurring units.

Namely, the polymer may further comprise recurring units derived fromcarbon-to-carbon double bond-bearing monomers other than theabove-described ones, for example, substituted acrylic acid esters suchas methyl methacrylate, methyl crotonate, dimethyl maleate and dimethylitaconate, unsaturated carboxylic acids such as maleic acid, fumaricacid, and itaconic acid, cyclic olefins such as norbornene, norbornenederivatives, and tetracyclo[6.2.1.1^(3,6)0.0^(2,7)]dodecene derivatives,unsaturated acid anhydrides such as itaconic anhydride, styrene,4-hydroxystyrene, 4-hydroxystyrene derivatives whose hydroxyl group isprotected, and other monomers. Also, hydrogenated products ofring-opening metathesis polymerization (ROMP) polymers as described inJP-A 2003-066612 may be used.

The polymer generally has a weight average molecular weight (Mw) of1,000 to 500,000, and preferably 3,000 to 100,000, as measured by gelpermeation chromatography (GPC) using polystyrene standards. Outside therange, there may result an extreme drop of etch resistance, and a dropof resolution due to difficulty to gain a dissolution rate differencebefore and after exposure.

The general method of synthesizing the polymer is, for example, bydissolving one or more unsaturated bond-bearing monomers in an organicsolvent, adding a radical initiator, and effecting heat polymerization.Reference may be made to many documents including JP-A 2005-264103. JP-A2010-077404 describes the synthesis of a polymer comprisingcopolymerized units having a triphenylsulfonium salt-containing compoundwhose anion is bound to the polymer backbone, which method is similar tothe above-mentioned one.

However, if the inventive polymer is synthesized by the above method,elimination of acid labile groups on copolymerizing units of formula (4)can occur under certain temperature and reaction conditions. This isprobably because the cation structure of the sulfonium salt is unstableand decomposed in part under such conditions. It is presumed that duringpolymerization reaction, copolymerizing units of formula (2a) or (2b)are partially decomposed to generate sulfonic acid, by which acid labilegroups on copolymerizing units of formula (4) are eliminated, failing toproduce the desired polymer.

The method for preparing the inventive polymer can avoid the aboveproblem. According to the invention, the desired polymer is prepared byreacting a polymer comprising recurring units having the general formula(6a) or (6b) with a sulfonium salt having the general formula (7).

Herein R¹ is hydrogen, fluorine, methyl or trifluoromethyl. R^(1a) toR^(1m) are each independently hydrogen or a C₁-C₂₀ straight, branched orcyclic, monovalent hydrocarbon group which may be substituted with orseparated by a heteroatom. L and L¹ each are a single bond or a C₁-C₂₀straight, branched or cyclic, divalent hydrocarbon group which may besubstituted with or separated by a heteroatom. X is a C₁-C₅ divalentalkylene group in which some or all hydrogen atoms may be substituted byfluorine atoms. A is hydrogen or trifluoromethyl, and n is 0 or 1. Za⁺is a lithium ion, sodium ion, potassium ion, or ammonium cation of thegeneral formula (8):

(R³)₄N⁺  (8)

wherein R³ is each independently hydrogen, a substituted orunsubstituted, C₁-C₁₀ straight, branched or cyclic alkyl, alkenyl oroxoalkyl group, or substituted or unsubstituted C₆-C₁₈ aryl, aralkyl oraryloxoalkyl group, or any two or more of R³ may bond together to form aring with N. Xa⁻ is an anion.

Since the cation structure is introduced into the sulfonium salt afterthe polymerization step that can induce decomposition, the inventivemethod is successful in producing the desired polymer without risks ofdecomposition of the sulfonium salt and elimination of acid labilegroups.

In formulae (6a) and (6b), R¹, L, L¹, X, A, and n are as defined above.Za⁺ is a lithium ion, sodium ion, potassium ion, or ammonium cation ofthe general formula (8). Xa⁻ is an anion.

(R³)₄N⁺  (8)

In formula (8), R³ is each independently hydrogen, a substituted orunsubstituted, C₁-C₁₀ straight, branched or cyclic alkyl, alkenyl oroxoalkyl group, or substituted or unsubstituted C₆-C₁₈ aryl, aralkyl oraryloxoalkyl group, or any two or more of R³ may bond together to form aring with the nitrogen atom. Suitable alkyl groups include methyl,ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl, hexyl,heptyl, octyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclopropylmethyl,4-methylcyclohexyl, cyclohexylmethyl, norbornyl, and adamantyl. Suitablealkenyl groups include allyl, propenyl, butenyl, hexenyl, andcyclohexenyl. Suitable oxoalkyl groups include 2-oxocyclopentyl,2-oxocyclohexyl, 2-oxopropyl, 2-oxoethyl, 2-cyclopentyl-2-oxoethyl,2-cyclohexyl-2-oxoethyl, and 2-(4-methylcyclohexyl)-2-oxoethyl. Suitablearyl groups include phenyl, 1-naphthyl, 2-naphthyl, thienyl,alkoxyphenyl groups such as 4-hydroxyphenyl, p-methoxyphenyl,m-methoxyphenyl, o-methoxyphenyl, p-ethoxyphenyl, p-tert-butoxyphenyl,and m-tert-butoxyphenyl, alkylphenyl groups such as 2-methylphenyl,3-methylphenyl, 4-methylphenyl, ethylphenyl, 4-tert-butylphenyl,4-butylphenyl, and 2,4-dimethylphenyl; alkylnaphthyl groups such as1-(4-methyl)naphthyl and 2-(6-methyl)naphthyl; and alkoxynaphthyl groupssuch as 1-(4-methoxy)naphthyl and 2-(6-methoxy)naphthyl. Suitablearalkyl groups include benzyl, 1-phenylethyl and 2-phenylethyl. Suitablearyloxoalkyl groups are 2-aryl-2-oxoethyl groups including2-phenyl-2-oxoethyl, 2-(1-naphthyl)-2-oxoethyl, and2-(2-naphthyl)-2-oxoethyl. Also included are substituted forms of theforegoing groups in which one or more hydrogen atoms are substituted bya heteroatom or atoms such as oxygen, sulfur, nitrogen, and halogen. Asa result of substitution, a hydroxyl, cyano, carbonyl, ether bond, esterbond, sulfonic acid ester bond, carbonate bond, lactone ring, sultonering, carboxylic anhydride or haloalkyl group may form.

Where two or more of R³ bond together to form a cyclic structure withthe nitrogen atom, suitable structures include piperidine, morpholine,pyridine, quinoline, acridine, imidazole, and benzimidazole, in whichthe nitrogen atom may be protonated or alkylated.

Examples of the ammonium cation of formula (8) include ammonium,trimethylammonium, tetramethylammonium, triethylammonium,tributylammonium, tetrabutylammonium, trioctylammonium, anilinium,2,6-dimethylanilinium, N,N-dimethylanilinium, benzyltrimethylammonium,bezyltriethylammonium, benzyltripropylammonium,N-benzyl-N,N-dimethylanilinium, andN-(p-methoxy)benzyl-N,N-dimethylanilinium. Inter alia, trimethlammonium,tetramethylammonium, triethylammonium, anilinium, andbenzyltrimethylammonium are preferred.

In formula (7), R^(1a) to R^(1m) are as exemplified above. Xa⁻ is ananion, which may be a conjugate base of a mineral acid or organic acid.Preferred are conjugate bases of methylsulfuric acid, methanesulfonicacid, paratoluenesulfonic acid, and trifluoromethanesulfonic acid, aswell as I⁻, Br⁻, Cl⁻, BF₄ ⁻, and ClO₄ ⁻.

For the synthesis of the polymer comprising recurring units havingformula (6a) or (6b), the standard radical polymerization method may beapplied as alluded to above. Also, the monomer corresponding to therecurring unit having formula (6a) or (6b) may be synthesized byreferring to the synthesis method described in JP-A 2007-304490, forexample. The sulfonium salt having formula (7) may be synthesized byreferring to the synthesis method described in J. Org. Chem., 1978, 43,3055-3058, for example.

Described below is the final step of the polymer preparation method.Once the polymer comprising recurring units having formula (6a) or (6b)is synthesized, it is mixed with the sulfonium salt having formula (7)in water and an organic solvent separable from water. From the mixture,the organic layer is taken out. The organic layer thus separated iswashed with water, if desired. Finally, the desired polymer is recoveredfrom the organic layer by such operation as concentration orcrystallization. The organic solvent used herein is not particularlylimited as long as it can be separated from water and the polymer can bedissolved therein. Inter alia, ketone solvents such as methyl ethylketone and methyl isobutyl ketone and halogenated solvents such asdichloromethane and chloroform are preferred.

Alternatively, the polymer comprising recurring units having formula(6a) or (6b) and the alkylsulfonium salt having formula (7) aredissolved in an organic solvent whereupon the desired polymer may berecovered by crystallization using water and organic solvents. Typicallyan alcohol is used as the good solvent and water is used as the poorsolvent. The aforementioned exchange reactions are merely exemplary, andthe method of preparing the inventive polymer is not limited thereto.

The method of preparing the inventive polymer is quite advantageous inthat the desired polymer can be produced without any risk ofdecomposition, because a sulfonium cation is introduced under mildconditions without a need for heating, nucleophilic compound or radicalinitiator.

While the polymer comprises recurring units derived from monomers, themolar fractions of respective units preferably fall in the followingrange (mol %), but are not limited thereto,

(I) 0.2 to 100 mol %, more preferably 0.5 to 50 mol %, and even morepreferably 0.5 to 30 mol % of recurring units having formula (2a) or(2b),(II) 0 to 50 mol %, more preferably 5 to 45 mol %, and even morepreferably 10 to 40 mol % of constituent units of at least one typehaving formula (3),(III) 0 to 50 mol %, more preferably 5 to 45 mol %, and even morepreferably 10 to 40 mol % of constituent units of at least one typehaving formula (4),(IV) 0 to 50 mol %, more preferably 5 to 45 mol %, and even morepreferably 10 to 40 mol % of constituent units of at least one typehaving formula (5), and optionally,(V) 0 to 99.8 mol %, more preferably 0 to 70 mol %, and even morepreferably 0 to 50 mol % of constituent units of at least one typederived from another monomer(s).

The polymer is not limited to one type and a mixture of two or morepolymers may be added. The use of plural polymers allows for easyadjustment of resist properties.

Resist Composition

A further embodiment of the invention is a resist composition comprising(A) a polymer comprising recurring units having formula (2a) or (2b) asessential component. The resist composition may further comprise (B) aphotoacid generator capable of generating an acid upon exposure, (C) aquencher, and (D) an organic solvent. Optionally, the resist compositionmay further comprise (E) a surfactant which is insoluble orsubstantially insoluble in water and soluble in alkaline developer,and/or a surfactant which is insoluble or substantially insoluble inwater and alkaline developer (hydrophobic resin), and (F) an organicacid derivative and/or fluorinated alcohol.

(B) Photoacid Generator

The PAG used herein may be any compound capable of generating an acidupon exposure to high-energy radiation including UV, DUV, EB, EUV,x-ray, excimer laser, γ-ray, and synchrotron radiation. Suitable PAGsinclude sulfonium salts, iodonium salts, sulfonyldiazomethane,N-sulfonyloxydicarboxyimide, O-arylsulfonyloxime, andO-alkylsulfonyloxime generators. The acid generators may be used aloneor in admixture of two or more.

Sulfonium salts are salts of sulfonium cations with sulfonates,bis(substituted alkylsulfonyl)imides and tris(substitutedalkylsulfonyl)methides. Suitable sulfonium cations include those cationshaving the general formula (9).

S⁺(R³³R⁴⁴R⁵⁵)  (9)

Herein R³³, R⁴⁴ and R⁵⁵ are each independently a substituted orunsubstituted, straight, branched or cyclic C₁-C₁₀ alkyl, alkenyl oroxoalkyl group or a substituted or unsubstituted C₆-C₁₈ aryl, aralkyl oraryloxoalkyl group, or any two of R³³, R⁴⁴ and R⁵⁵ may bond together toform a ring with the sulfur atom in the formula.

Of the groups represented by R³³, R⁴⁴ and R⁵⁵, suitable alkyl groupsinclude methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl,cyclopropyl, cyclopentyl, cyclohexyl, cyclopropylmethyl,4-methylcyclohexyl, cyclohexylmethyl, norbornyl, and adamantyl. Suitablealkenyl groups include vinyl, allyl, propenyl, butenyl, hexenyl, andcyclohexenyl. Suitable oxoalkyl groups include 2-oxocyclopentyl,2-oxocyclohexyl, 2-oxopropyl, 2-oxoethyl, 2-cyclopentyl-2-oxoethyl,2-cyclohexyl-2-oxoethyl, and 2-(4-methylcyclohexyl)-2-oxoethyl. Suitablearyl groups include phenyl, naphthyl and thienyl, hydroxyphenyl groupssuch as 4-hydroxyphenyl, alkoxyphenyl groups such as 4-methoxyphenyl,3-methoxyphenyl, 2-methoxyphenyl, 4-ethoxyphenyl, 4-tert-butoxyphenyl,and 3-tert-butoxyphenyl, alkylphenyl groups such as 2-methylphenyl,3-methylphenyl, 4-methylphenyl, 4-ethylphenyl, 4-tert-butylphenyl,4-n-butylphenyl, and 2,4-dimethylphenyl, alkylnaphthyl groups such asmethylnaphthyl and ethylnaphthyl, alkoxynaphthyl groups such asmethoxynaphthyl and ethoxynaphthyl, dialkylnaphthyl groups such asdimethylnaphthyl and diethylnaphthyl, and dialkoxynaphthyl groups suchas dimethoxynaphthyl and diethoxynaphthyl. Suitable aralkyl groupsinclude benzyl, 1-phenylethyl, and 2-phenylethyl. Suitable aryloxoalkylgroups are 2-aryl-2-oxoethyl groups including 2-phenyl-2-oxoethyl,2-(1-naphthyl)-2-oxoethyl, and 2-(2-naphthyl)-2-oxoethyl. In thesehydrocarbon groups, one or more hydrogen atoms may be substituted byfluorine or hydroxyl.

Alternatively, any two of R³³, R⁴⁴ and R⁵⁵ bond together to form a ringwith the sulfur atom in the formula. Exemplary ring structures are givenbelow.

Herein R⁶⁶ is as defined and illustrated for R³³, R⁴⁴ and R⁵⁵.

As the anion of the sulfonium salt, exemplary sulfonates includetrifluoromethanesulfonate, pentafluoroethanesulfonate,heptafluoropropanesulfonate, nonafluorobutanesulfonate,tridecafluorohexanesulfonate, 2,2,2-trifluoroethanesulfonate,pentafluorobenzenesulfonate, 1,1-difluoro-2-naphthylethanesulfonate,1,1,2,2-tetrafluoro-2-(norbornan-2-yl)ethanesulfonate,1,1,2,2-tetrafluoro-2-(tetracyclo[6.2.1.1^(3,6)0.0^(2,7)]dodec-3-en-8-yl)ethanesulfonate,2-benzoyloxy-1,1,3,3,3-pentafluoropropanesulfonate,1,1-difluoro-2-tosyloxyethanesulfonate,adamantanemethoxycarbonyldifluoromethanesulfonate,1-(3-hydroxymethyladamantane)methoxycarbonyldifluoro-methanesulfonate,methoxycarbonyldifluoromethanesulfonate,1-(hexahydro-2-oxo-3,5-methano-2H-cyclopenta[b]furan-6-yl-oxycarbonyl)difluoromethanesulfonate,and 4-oxo-1-adamantyloxycarbonyldifluoromethanesulfonate. Exemplarybis(substituted alkylsulfonyl)imides includebis(trifluoromethylsulfonyl)imide, bis(pentafluoroethylsulfonyl)imide,bis(heptafluoropropylsulfonyl)imide, andperfluoro(1,3-propylenebissulfonyl)imide. A typical tris(substitutedalkylsulfonyl)methide is tris(trifluoromethylsulfonyl)methide. Sulfoniumsalts based on combination of the foregoing examples are included.

Examples of the iodonium salt, N-sulfonyloxydicarboxyimide,O-arylsulfonyloxime, and O-alkylsulfonyloxime (or oximesulfonate) acidgenerators are described in JP-A 2009-269953 (U.S. Pat. No. 8,114,571).

Preferred examples of the other PAG include triphenylsulfoniumnonafluorobutanesulfonate, triphenylsulfoniumbis(trifluoromethylsulfonyl)imide, triphenylsulfoniumperfluoro(1,3-propylenebissulfonyl)imide, triphenylsulfoniumtris(trifluoromethanesulfonyl)methide,N-nonafluorobutanesulfonyloxy-1,8-naphthalenedicarboxyimide,2-(2,2,3,3,4,4-hexafluoro-1-(nonafluorobutylsulfonyloxy-imino)butyl)fluorene,and2-(2,2,3,3,4,4,5,5-octafluoro-1-(nonafluorobutylsulfonyloxy-imino)pentyl)fluorene.

The preferred structure of PAG includes compounds having the generalformula (P1).

Herein R⁷⁷ is hydrogen or trifluoromethyl, R⁸⁸ is a C₁-C₃₀ alkyl,alkenyl or aralkyl group which may contain a heteroatom, R³³, R⁴⁴ andR⁵⁵ are as defined above.

In formula (P1), R⁸⁸ is a C₁-C₃₀ alkyl, alkenyl or aralkyl groupoptionally containing a heteroatom. Suitable heteroatoms contained inR⁸⁸ include oxygen, nitrogen, sulfur and halogen atoms, with oxygenbeing preferred. The C₁-C₃₀ alkyl, alkenyl or aralkyl group of R⁸⁸ maybe straight, branched or cyclic while it is preferred for achieving ahigh resolution sufficient to form a fine size pattern that these groupshave 6 to 30 carbon atoms. It is undesirable that R⁸⁸ be aryl becausethe resulting resist pattern may have less smooth sidewalls. Exemplarygroups of R⁸⁸ include, but are not limited to, methyl, ethyl, propyl,isopropyl, butyl, isobutyl, s-butyl, tert-butyl, pentyl, neopentyl,cyclopentyl, hexyl, cyclohexyl, 3-cyclohexenyl, heptyl, 2-ethylhexyl,nonyl, undecyl, tridecyl, pentadecyl, heptadecyl, 1-adamantyl,2-adamantyl, 1-adamantylmethyl, norbornyl, norbornylmethyl,tricyclodecanyl, tetracyclododecanyl, tetracyclododecanylmethyl,dicyclohexylmethyl, eicosanyl, allyl, benzyl, diphenylmethyl,tetrahydrofuryl, methoxymethyl, ethoxymethyl, methylthiomethyl,acetamidomethyl, trifluoromethyl, (2-methoxyethoxy)methyl,acetoxymethyl, 2-carboxy-1-cyclohexyl, 2-oxopropyl, 4-oxo-1-adamantyl,and 3-oxocyclohexyl.

With respect to the synthesis of the sulfonium salt having formula (P1),reference may be made to JP-A 2007-145797, 2008-106045, 2009-007327, and2009-258695, for example.

Illustrative examples of the preferred PAG are given below.

Herein Ac stands for acetyl and Ph stands for phenyl.

An appropriate amount of the PAG having formula (P1) added is 0 to 40parts by weight, and if added, 0.1 to 40 parts, and more preferably 0.1to 20 parts by weight per 100 parts by weight of the polymer as baseresin. Too high a proportion of the PAG may give rise to problems suchas degraded resolution and foreign particles after resist development orduring resist film stripping. The PAG having formula (P1) may be usedalone or in admixture of two or more or in admixture with another PAG.When the other PAG is added, its amount is arbitrary as long as theobjects of the invention are not compromised. Typically the amount ofthe other PAG is 0 to 20 parts, preferably 0.1 to 10 parts by weight per100 parts by weight of the polymer.

Notably, the resist composition comprises as base resin (A) a polymercomprising recurring units of sulfonium salt having formula (2a) or(2b), which functions as PAG. Therefore, it is unnecessary to add PAG(B) although it is acceptable to use one or more PAGs (B) in combinationwith base resin (A).

It is noted that an acid diffusion controlling function may be providedwhen two or more PAGs are used in admixture provided that one PAG is anonium salt capable of generating a weak acid. Specifically, in a systemusing a mixture of an onium salt capable of generating a strong acid(e.g., fluorinated sulfonic acid) and an onium salt capable ofgenerating a weak acid (e.g., non-fluorinated sulfonic acid orcarboxylic acid), if the strong acid generated from the PAG uponexposure to high-energy radiation collides with the unreacted onium salthaving a weak acid anion, then a salt exchange occurs whereby the weakacid is released and an onium salt having a strong acid anion is formed.In this course, the strong acid is exchanged into the weak acid having alow catalysis, incurring apparent deactivation of the acid for enablingto control acid diffusion.

If the PAG capable of generating a strong acid is an onium salt, anexchange from the strong acid generated upon exposure to high-energyradiation to a weak acid as above can take place, but it never happensthat the weak acid generated upon exposure to high-energy radiationcollides with the unreacted onium salt capable of generating a strongacid to induce a salt exchange. This is because of a likelihood of anonium cation forming an ion pair with a stronger acid anion.

(C) Quencher

The quencher (C) may be a compound capable of suppressing the rate ofdiffusion when the acid generated by the PAG diffuses within the resistfilm. The inclusion of quencher facilitates adjustment of resistsensitivity and holds down the rate of acid diffusion within the resistfilm, resulting in better resolution. In addition, it suppresses changesin sensitivity following exposure and reduces substrate and environmentdependence, as well as improving the exposure latitude and the patternprofile. The inclusion of quencher is also effective for improvingadhesion to the substrate.

Examples of suitable quenchers include primary, secondary, and tertiaryaliphatic amines, mixed amines, aromatic amines, heterocyclic amines,nitrogen-containing compounds with carboxyl group, nitrogen-containingcompounds with sulfonyl group, nitrogen-containing compounds withhydroxyl group, nitrogen-containing compounds with hydroxyphenyl group,alcoholic nitrogen-containing compounds, amide derivatives, imidederivatives, carbamate derivatives, and ammonium salts. Of these,preferred are tertiary amines, amine oxides, benzimidazoles, andanilines having a polar functional group such as ether, carbonyl, esteror alcohol.

Preferred tertiary amines include 2-morpholinoethyl esters of straight,branched or cyclic C₂-C₂₀ aliphatic carboxylic acids and trialkylamineshaving a straight, branched or cyclic C₂-C₁₀ alkyl moiety. Also includedare substituted forms of these amines in which some carbon-bondedhydrogen atoms are replaced by hydroxyl groups. These amines may have anether or ester linkage. Examples include 2-morpholinoethyl2-methoxyacetate, 2-morpholinoethyl 2-(2-methoxyethoxy)acetate,2-morpholinoethyl 2-[2-(2-methoxyethoxy)ethoxy]acetate,2-morpholinoethyl hexanoate, 2-morpholinoethyl octanoate,2-morpholinoethyl decanoate, 2-morpholinoethyl laurate,2-morpholinoethyl myristate, 2-morpholinoethyl palmitate,2-morpholinoethyl stearate, 2-morpholinoethyl cyclohexanecarboxylate,2-morpholinoethyl adamantanecarboxylate,4-[2-[(2-methoxyethoxy)methoxy]ethyl]morpholine,4-[2-[2-(2-methoxyethoxy)ethoxy]ethyl]morpholine,4-[2-[2-(2-butoxyethoxy)ethoxy]ethyl]morpholine,tris(2-methoxymethoxyethyl)amine, tris{2-(2-methoxyethoxy)ethyl}amine,tris{2-(2-methoxyethoxymethoxy)ethyl}amine,tris{2-(1-methoxyethoxy)ethyl}amine, tris{2-(1-ethoxyethoxy)ethyl}amine,tris(2-acetoxyethyl)amine, tris(2-propionyloxyethyl)amine,tris(2-butyryloxyethyl)amine, tris(2-isobutyryloxyethyl)amine,tris(2-valeryloxyethyl)amine, and tris(2-pivaloyloxyethyl)amine.

Preferred examples of the benzimidazoles include benzimidazole,2-phenylbenzimidazole, 1-(2-acetoxyethoxy)benzimidazole,1-[2-(methoxymethoxy)ethyl]benzimidazole,1-[2-(methoxymethoxy)ethyl]-2-phenylbenzimidazole, and1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)benzimidazole.

Preferred examples of the anilines include aniline, N-methylaniline,N-ethylaniline, N-propylaniline, N,N-dimethylaniline,N,N-bis(hydroxyethyl)aniline, 2-methylaniline, 3-methylaniline,4-methylaniline, ethylaniline, propylaniline, dimethylaniline,2,6-diisopropylaniline, trimethylaniline, 2-nitroaniline,3-nitroaniline, 4-nitroaniline, 2,4-dinitroaniline, 2,6-dinitroaniline,3,5-dinitroaniline, and N,N-dimethyltoluidine.

Also included are primary and secondary amines which have been protectedwith tert-butoxycarbonyl (tBOC). Those compounds described in JP-A2007-298569 and JP-A 2010-020204 are also useful.

The quenchers may be used alone or in admixture of two or more. Thequencher is preferably used in an amount of 0.001 to 8 parts, morepreferably 0.01 to 4 parts by weight per 100 parts by weight of the baseresin. Less than 0.001 part of the quencher may achieve no additioneffect whereas more than 8 parts may lead to too low a sensitivity.

(D) Organic Solvent

The organic solvent (D) used herein may be any organic solvent in whichthe polymer (or base resin), PAG, quencher, and other components aresoluble. Illustrative, non-limiting, examples of the organic solventinclude ketones such as cyclohexanone and methyl amyl ketone; alcoholssuch as 3-methoxybutanol, 3-methyl-3-methoxybutanol,1-methoxy-2-propanol, and 1-ethoxy-2-propanol; ethers such as propyleneglycol monomethyl ether, propylene glycol monoethyl ether, propyleneglycol dimethyl ether, and diethylene glycol dimethyl ether; esters suchas propylene glycol monomethyl ether acetate (PGMEA), propylene glycolmonoethyl ether acetate, ethyl lactate, ethyl pyruvate, butyl acetate,methyl 3-methoxypropionate, ethyl 3-ethoxypropionate, tert-butylacetate, tert-butyl propionate, and propylene glycol mono-tert-butylether acetate; and lactones such as γ-butyrolactone, which may be usedalone or in combinations of two or more. Of the above organic solvents,it is recommended to use 1-ethoxy-2-propanol, PGMEA, cyclohexanone,γ-butyrolactone, and mixtures thereof because the acid generator is mostsoluble therein.

An appropriate amount of the organic solvent used is 200 to 5,000 parts,more preferably 400 to 3,000 parts by weight per 100 parts by weight ofthe base resin.

(E) Surfactant

Component (E) is a surfactant which is insoluble or substantiallyinsoluble in water and soluble in alkaline developer, and/or asurfactant which is insoluble or substantially insoluble in water andalkaline developer (hydrophobic resin). The surfactant (E) may be addedto the resist composition. Reference should be made to those compoundsdefined as component (S) in JP-A 2010-215608 and JP-A 2011-016746.

While many examples of the surfactant which is insoluble orsubstantially insoluble in water and alkaline developer are described inthese patent documents, preferred examples are FC-4430, Surflon S-381,Surfynol E1004, KH-20 and KH-30, which may be used alone or inadmixture. Partially fluorinated oxetane ring-opened polymers having thestructural formula (surf-1) are also useful.

It is provided herein that R, Rf, A, B, C, m, and n are applied to onlyformula (surf-1), independent of their descriptions other than for thesurfactant. R is a di- to tetra-valent C₂-C₅ aliphatic group. Exemplarydivalent groups include ethylene, 1,4-butylene, 1,2-propylene,2,2-dimethyl-1,3-propylene and 1,5-pentylene. Exemplary tri- andtetra-valent groups are shown below.

Herein the broken line denotes a valence bond. These formulae arepartial structures derived from glycerol, trimethylol ethane,trimethylol propane, and pentaerythritol, respectively. Of these,1,4-butylene and 2,2-dimethyl-1,3-propylene are preferably used.

Rf is trifluoromethyl or pentafluoroethyl, and preferablytrifluoromethyl. The letter m is an integer of 0 to 3, n is an integerof 1 to 4, and the sum of m and n, which represents the valence of R, isan integer of 2 to 4. A is equal to 1, B is an integer of 2 to 25, and Cis an integer of 0 to 10. Preferably, B is an integer of 4 to 20, and Cis 0 or 1. Note that the above structural formula does not prescribe thearrangement of respective constituent units while they may be arrangedeither in blocks or randomly. For the preparation of surfactants in theform of partially fluorinated oxetane ring-opened polymers, referenceshould be made to U.S. Pat. No. 5,650,483, for example.

The surfactant which is insoluble or substantially insoluble in waterand soluble in alkaline developer is useful when ArF immersionlithography is applied to the resist composition in the absence of aresist protective film. In this embodiment, the surfactant has apropensity to segregate on the resist surface after spin coating forachieving a function of minimizing water penetration or leaching. Thesurfactant is also effective for preventing water-soluble componentsfrom being leached out of the resist film for minimizing any damage tothe exposure tool. The surfactant becomes solubilized during alkalinedevelopment following exposure and PEB, and thus forms few or no foreignparticles which become defects. The preferred surfactant is a polymericsurfactant which is insoluble or substantially insoluble in water, butsoluble in alkaline developer, also referred to as “hydrophobic resin”in this sense, and especially which is water repellent and enhanceswater slippage. Suitable polymeric surfactants are shown below.

Herein R¹¹⁴ is each independently hydrogen, fluorine, methyl ortrifluoromethyl. R¹¹⁵ is each independently hydrogen or a straight,branched or cyclic C₁-C₂₀ alkyl or fluoroalkyl group, or two R¹¹⁵ in acommon monomer may bond together to form a ring with the carbon atom towhich they are attached, and in this event, they together represent astraight, branched or cyclic C₂-C₂₀ alkylene or fluoroalkylene group.R¹¹⁶ is fluorine or hydrogen, or R¹¹⁶ may bond with R¹¹⁷ to form anon-aromatic ring of 3 to 10 carbon atoms in total with the carbon atomto which they are attached. R¹¹⁷ is a straight, branched or cyclic C₁-C₆alkylene group in which at least one hydrogen atom may be substituted bya fluorine atom. R¹¹⁸ is a straight or branched C₁-C₁₀ alkyl group inwhich at least one hydrogen atom is substituted by a fluorine atom.Alternatively, R¹¹⁷ and R¹¹⁸ may bond together to form a non-aromaticring with the carbon atoms to which they are attached. In this event,R¹¹⁷, R¹¹⁸ and the carbon atoms to which they are attached togetherrepresent a trivalent organic group of 2 to 12 carbon atoms in total.R¹¹⁹ is a single bond or a C₁-C₄ alkylene. R¹²° is each independently asingle bond, —O—, or —CR¹¹⁴R¹¹⁴—. R¹²¹ is a straight or branched C₁-C₄alkylene group, or may bond with R¹¹⁵ within a common monomer to form aC₃-C₆ non-aromatic ring with the carbon atom to which they are attached.R¹²² is 1,2-ethylene, 1,3-propylene, or 1,4-butylene. Rf is a linearperfluoroalkyl group of 3 to 6 carbon atoms, typically3H-perfluoropropyl, 4H-perfluorobutyl, 5H-perfluoropentyl, or6H-perfluorohexyl. X² is each independently —C(C═O)—O—, —O—, or—C(C═O)—R¹²³—C(C═O)—O—. R¹²³ is a straight, branched or cyclic C₁-C₁₀alkylene group. The subscripts are in the range:

0≦(a′−1)<1, 0≦(a′−2)<1, 0≦(a′−3)<1,

0<(a′−1)+(a′−2)+(a′−3)<1, 0≦b′<1, 0≦c′<1, and

0<(a′−1)+(a′−2)+(a′−3)+b′+c′≦1.

Examples of these units are shown below.

For the surfactant which is insoluble or substantially insoluble inwater and soluble in alkaline developer, reference may be made to JP-A2008-122932, 2010-134012, 2010-107695, 2009-276363, 2009-192784,2009-191151, 2009-98638, 2011-250105, and 2011-42789.

There may also be added styrene and vinylnaphthalene copolymers asdescribed in JP 4900603 and JP-A 2008-203452. These polymers tend tosegregate at the resist film surface after spin coating and are thuseffective for reducing outgassing components from within the resist filmduring the exposure step. They are thus advantageous in the EUVlithography where outgassing suppression is requisite.

The polymeric surfactant preferably has a Mw of 1,000 to 50,000, morepreferably 2,000 to 20,000 as measured by GPC versus polystyrenestandards. A surfactant with a Mw outside the range may be lesseffective for surface modification and cause development defects. Thepolymeric surfactant is preferably formulated in an amount of 0.001 to20 parts, and more preferably 0.01 to 10 parts by weight per 100 partsby weight of the base resin. Reference should also be made to JP-A2010-215608.

(F) Organic Acid Derivative and/or Fluorinated Alcohol

To the resist composition, a compound which is decomposed with an acidto generate another acid, that is, acid amplifier compound may be added.For these compounds, reference should be made to JP-A 2009-269953 and2010-215608. In the resist composition, an appropriate amount of theacid amplifier compound is up to 2 parts, and especially up to 1 part byweight per 100 parts by weight of the base resin. Excessive amounts ofthe acid amplifier compound make diffusion control difficult, leading todegradation of resolution and pattern profile.

Optionally, an organic acid derivative or a compound having a Mw of upto 3,000 which changes its solubility in alkaline developer under theaction of an acid, also referred to as dissolution inhibitor, may beadded. Reference may be made to JP-A 2009-269953 and 2010-215608.

Patterning Process

A further embodiment of the invention is a pattern forming process usingthe resist composition defined above. A pattern may be formed from theresist composition using any well-known lithography process. Thepreferred process includes at least the steps of forming a resist filmon a substrate, exposing it to high-energy radiation, and developing itin a developer.

First the resist composition is applied onto a substrate for integratedcircuitry fabrication (e.g., Si, SiO₂, SiN, SiON, TiN, WSi, BPSG, SOG,organic antireflective film, etc.) or a substrate for mask circuitryfabrication (e.g., Cr, CrO, CrON, MoSi, etc.) by a suitable coatingtechnique such as spin coating. The coating is prebaked on a hot plateat a temperature of 60 to 150° C. for 1 to 10 minutes, preferably 80 to140° C. for 1 to 5 minutes. The resulting resist film is generally 0.05to 2.0 μm thick. Through a photomask having a desired pattern disposedover the substrate, the resist film is then exposed to high-energyradiation such as deep-UV, excimer laser or x-ray, or electron beam inan exposure dose preferably in the range of 1 to 200 mJ/cm², morepreferably 10 to 100 mJ/cm². Alternatively, pattern formation may beperformed by writing with an electron beam directly (not through amask). Light exposure may be done by a conventional lithography processor in some cases, by an immersion lithography process of providingliquid impregnation, typically water, between the projection lens ormask and the resist film. In the case of immersion lithography, aprotective film which is insoluble in water may be used. The resist filmis then baked (PEB) on a hot plate at 60 to 150° C. for 1 to 5 minutes,and preferably at 80 to 140° C. for 1 to 3 minutes. Finally, developmentis carried out using as the developer an aqueous alkaline solution, suchas a 0.1 to 5 wt %, preferably 2 to 3 wt %, aqueous solution oftetramethylammonium hydroxide (TMAH), this being done by a conventionalmethod such as dip, puddle, or spray development for a period of 0.1 to3 minutes, and preferably 0.5 to 2 minutes. In this way the desiredpattern is formed on the substrate. Of the various types of high-energyradiation that may be used, the resist composition of the invention isbest suited to fine pattern formation with, in particular, deep-UV orexcimer laser having a wavelength of 250 to 190 nm, x-ray, or EB. Thedesired pattern may not be obtainable outside the upper and lower limitsof the above range.

While the water-insoluble protective film which is used in the immersionlithography serves to prevent any components from being leached out ofthe resist film and to improve water slippage at the film surface, it isgenerally divided into two types. The first type is an organicsolvent-strippable protective film which must be stripped, prior toalkaline development, with an organic solvent in which the resist filmis not dissolvable. The second type is an alkali-soluble protective filmwhich is soluble in an alkaline developer so that it can be removedsimultaneously with the removal of solubilized regions of the resistfilm. The protective film of the second type is preferably of a materialcomprising a polymer having a 1,1,1,3,3,3-hexafluoro-2-propanol residue(which is insoluble in water and soluble in an alkaline developer) as abase in an alcohol solvent of at least 4 carbon atoms, an ether solventof 8 to 12 carbon atoms or a mixture thereof. Alternatively, theaforementioned surfactant which is insoluble in water and soluble in analkaline developer may be dissolved in an alcohol solvent of at least 4carbon atoms, an ether solvent of 8 to 12 carbon atoms or a mixturethereof to form a material from which the protective film of the secondtype is formed.

Any desired step may be added to the pattern forming process. Forexample, after a photoresist film is formed, a step of rinsing with purewater (post-soaking) may be introduced to extract the acid generator orthe like from the film surface or wash away particles. After exposure, astep of rinsing (post-soaking) may be introduced to remove any waterremaining on the film after exposure.

The technique enabling the ArF lithography to survive to the 32-nm nodeis a double patterning process. The double patterning process includes atrench process of processing an underlay to a 1:3 trench pattern by afirst step of exposure and etching, shifting the position, and forming a1:3 trench pattern by a second step of exposure for forming a 1:1pattern; and a line process of processing a first underlay to a 1:3isolated left pattern by a first step of exposure and etching, shiftingthe position, processing a second underlay formed below the firstunderlay by a second step of exposure through the 1:3 isolated leftpattern, for forming a half-pitch 1:1 pattern.

In the pattern forming process, an alkaline aqueous solution, typicallyan aqueous solution of 0.1 to 5 wt %, more typically 2 to 3 wt % oftetramethylammonium hydroxide (TMAH) is often used as the developer. Thenegative tone development technique wherein the unexposed region isdeveloped and dissolved in an organic solvent is also applicable.

In the organic solvent development, the organic solvent used as thedeveloper is preferably selected from 2-octanone, 2-nonanone,2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone,diisobutyl ketone, methylcyclohexanone, acetophenone,methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate,amyl acetate, isoamyl acetate, butenyl acetate, phenyl acetate, propylformate, butyl formate, isobutyl formate, amyl formate, isoamyl formate,methyl valerate, methyl pentenoate, methyl crotonate, ethyl crotonate,methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isobutyllactate, amyl lactate, isoamyl lactate, methyl 2-hydroxyisobutyrate,ethyl 2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, benzylacetate, methyl phenylacetate, benzyl formate, phenylethyl formate,methyl 3-phenylpropionate, benzyl propionate, ethyl phenylacetate, and2-phenylethyl acetate. These organic solvents may be used alone or inadmixture of two or more.

Example

Examples and Comparative Examples are given below by way of illustrationand not by way of limitation. Mw is weight average molecular weight andMw/Mn is dispersity. All parts are by weight (pbw).

Synthesis Example 1-1 Synthesis of 9-oxo-10-phenyl-9H-thioxantheniumtrifluoromethanesulfonate (PAG-1)

A mixture of 1.1 g (5 mmol) of thioxanthone, 2.2 g (5 mmol) ofdiphenyliodonium trifluoromethanesulfonate, 46 mg (0.15 mmol) ofcopper(II) benzoate, and 11 g of chlorobenzene was heated and stirred at120° C. for 3 hours. At the end of stirring, the reaction solution wascooled to room temperature, and 20 g of diisopropyl ether was added forrecrystallization. The resulting solid was dried in vacuum, obtaining1.4 g (yield 65%) of the desired compound,9-oxo-10-phenyl-9H-thioxanthenium trifluoromethanesulfonate.

Time-of-Flight Mass Spectrometry (TOFMS; MALDI)

Positive M⁺289 (corresponding to (C₆H₅) (C₁₃H₈O)S⁺)

Negative M⁻149 (corresponding to CF₃SO₃ ⁻)

Synthesis Example 1-2 Synthesis of triethylammonium1,1,3,3,3-pentafluoro-2-hydroxypropane-1-sulfonate

According to the method described in JP-A 2007-304490, triethylammonium1,1,3,3,3-pentafluoro-2-(pivaloyloxy)-propane-1-sulfonate wassynthesized. According to the method described in JP-A 2007-145804, thepivaloyl group was subjected to hydrolysis (solvolysis), obtainingtriethylammonium 1,1,3,3,3-pentafluoro-2-hydroxypropane-1-sulfonate aswhile crystal.

Synthesis Example 1-3 Synthesis of triethylammonium1,1,3,3,3-pentafluoro-2-methacryloyloxypropane-1-sulfonate

Under ice cooling, 28 g (0.18 mol) of methacrylic anhydride was addeddropwise to a mixture of 79 g (0.16 mol) of triethylammonium1,1,3,3,3-pentafluoro-2-hydroxypropane-1-sulfonate in Synthesis Example1-2, 19 g (0.19 mol) of triethylamine, 0.10 g (0.8 mmol) ofN,N′-dimethylaminopyridine, and 400 g of methylene chloride. Thereafter,the mixture was stirred overnight at room temperature. Dilutehydrochloric acid was added to the reaction mixture to quench thereaction, from which the organic layer was taken out and washed withwater. After washing, the organic layer was concentrated, combined withmethyl isobutyl ketone, and concentrated again. The residue was washedwith diisopropyl ether, obtaining 45 g (yield 70%) of the desiredcompound, triethylammonium1,1,3,3,3-pentafluoro-2-methacryloyloxypropane-1-sulfonate.

Synthesis Example 1-4 Synthesis of benzyltrimethylammonium1,1,3,3,3-pentafluoro-2-hydroxypropane-1-sulfonate

According to the method described in JP-A 2007-304490,benzyltrimethylammonium1,1,3,3,3-pentafluoro-2-(pivaloyloxy)-propane-1-sulfonate wassynthesized. According to the method described in JP-A 2007-145804, thepivaloyl group was subjected to hydrolysis (solvolysis), obtainingbenzyltrimethylammonium1,1,3,3,3-pentafluoro-2-hydroxypropane-1-sulfonate as while crystal.

Synthesis Example 1-5 Synthesis of benzyltrimethylammonium1,1,3,3,3-pentafluoro-2-methacryloyloxypropane-1-sulfonate

According to the method described in JP-A 2008-133448,benzyltrimethylammonium1,1,3,3,3-pentafluoro-2-hydroxy-propane-1-sulfonate obtained inSynthesis Example 1-4 was converted to benzyltrimethylammonium1,1,3,3,3-pentafluoro-2-methacryloyloxypropane-1-sulfonate.

Synthesis Example 1-6 Synthesis of benzyltrimethylammonium1,1,3,3,3-pentafluoro-2-(3-methacryloyloxy-adamantane-1-carboxyloxy)propane-1-sulfonate

According to the method described in JP-A 2010-077404,benzyltrimethylammonium1,1,3,3,3-pentafluoro-2-hydroxy-propane-1-sulfonate obtained inSynthesis Example 1-4 was converted to benzyltrimethylammonium1,1,3,3,3-pentafluoro-2-(3-methacryloyloxy-adamantane-1-carboxyloxy)propane-1-sulfonate.

Synthesis Example 1-7 Synthesis of 9-oxo-10-phenyl-9H-thioxanthenium2-methacryloyloxy-1,1,3,3,3-pentafluoropropane-1-sulfonate (PAG-2)

A mixture of 1.3 g (3 mmol) of 9-oxo-10-phenyl-9H-thioxantheniumtrifluoromethanesulfonate obtained in Synthesis Example 1-1, 1.3 g (3mmol) of benzyltrimethyl-ammonium1,1,3,3,3-pentafluoro-2-methacryloyloxypropane-1-sulfonate obtained inSynthesis Example 1-5, 20 g of methyl isobutyl ketone, and 20 g of waterwas stirred at room temperature for 30 minutes. The organic layer wasextracted out and washed with water. After washing, the organic layerwas concentrated. Diisopropyl ether, 10 g, was added to the residue forrecrystallization. Subsequent filtration and vacuum drying gave 1.4 g(yield 80%) of the target compound, 9-oxo-10-phenyl-9H-thioxanthenium2-methacryloyloxy-1,1,3,3,3-pentafluoropropane-1-sulfonate.

The target compound was analyzed by spectroscopy, with the results shownbelow. FIGS. 1 and 2 are nuclear magnetic resonance spectra (¹H-NMR and¹⁹F-NMR/DMSO-d₆). In ¹H-NMR, a trace amount of water was observed.

Infrared Absorption Spectrum (IR (D-ATR)):

-   -   3089, 3068, 3025, 2966, 1732, 1668, 1586, 1570, 1450, 1441,        1379, 1316, 1301, 1280, 1255, 1175, 1144, 1116, 1070, 991, 961,        925, 903, 842, 808, 747, 729, 681, 639 cm⁻¹

TOFMS; MALDI

Positive M⁺289 (corresponding to (C₆H₅) (C₁₃H₂O)S⁺)

Negative M⁻297 (corresponding to CF₃CH(OCO—C₃H₅)CF₂SO₃ ⁻)

Synthesis Example 1-8 Synthesis of 9-oxo-10-phenyl-9H-thioxanthenium1,1,3,3,3-pentafluoro-2-(3-methacryloyloxy-adamantane-1-carbonyloxy)-propane-1-sulfonate(PAG-3)

The same procedure as Synthesis Example 1-7 was repeated aside fromusing 9-oxo-10-phenyl-9H-thioxanthenium trifluoro-methanesulfonateobtained in Synthesis Example 1-1 and benzyltrimethylammonium1,1,3,3,3-pentafluoro-2-(3-methacryl-oyloxy-adamantane-1-carbonyloxy)propane-1-sulfonateobtained in Synthesis Example 1-6. There was obtained9-oxo-10-phenyl-9H-thioxanthenium1,1,3,3,3-pentafluoro-2-(3-methacryloyloxy-adamantane-1-carbonyloxy)propane-1-sulfonate(PAG-3).

The target compound was analyzed by spectroscopy, with the results shownbelow. FIGS. 3 and 4 are NMR spectra (¹H-NMR and ¹⁹F-NMR/DMSO-d₆). In¹H-NMR, traces of residual solvents (methyl isobutyl ketone, diisopropylether and water) were observed.

IR (D-ATR):

-   -   3081, 2970, 2927, 2857, 1750, 1704, 1676, 1634, 1587, 1572,        1480, 1448, 1433, 1376, 1318, 1302, 1272, 1262, 1249, 1232,        1214, 1184, 1159, 1116, 1086, 1050, 1008, 991, 939, 925, 898,        863, 839, 807, 776, 750, 731, 686, 667, 639, 576 cm⁻¹

TOFMS; MALDI

Positive M⁺289 (corresponding to (C₆H₅)(C₁₃H₈O)S⁺)

Negative M⁻475 (corresponding to CF₃CH)(OCO—C₁₄H₁₉)CF₂SO₃ ⁻)

As comparative photoacid generators, compounds PAG-4 to PAG-6 of thefollowing structure were prepared. PAG-4 to PAG-6 could be synthesizedwith reference to JP-A 2007-145797 and 2008-133448, for example.

Synthesis Example 2-1 Synthesis of Polymer (P-0)

A nitrogen atmosphere flask was charged with 5.3 g of triethylammonium1,1,3,3,3-pentafluoro-2-methacryloyloxy-propane-1-sulfonate, 4.9 g of3-ethyl-3-exo-tetracyclo-[4.4.0.1^(2,5).1^(7,10)]dodecanyl methacrylate,2.1 g of 4-hydroxy-phenyl methacrylate, 4.0 g of4,8-dioxatricyclo[4.2.1.0^(3,7)]-nonan-5-on-2-yl methacrylate, 1.4 g of2,2′-azobisisobutyro-nitrile, 17.5 g of methyl ethyl ketone, and 17.5 gof cyclohexanone to form a monomer solution. Another nitrogen atmosphereflask was charged with 5.8 g of methyl ethyl ketone and 5.8 g ofcyclohexanone, which were heated at 80° C. with stirring. The monomersolution was added dropwise to the solvent mixture over 4 hours. Afterthe completion of dropwise addition, the polymerization solution wasstirred for 2 hours while keeping it at 80° C. The polymerizationsolution was cooled to room temperature, after which it was addeddropwise to a mixture of 20.0 g of methyl ethyl ketone and 180 g ofhexane. The copolymer precipitate was collected by filtration. Thecopolymer was washed twice with a mixture of 37.0 g of methyl ethylketone and 83.1 g of hexane. It was vacuum dried at 50° C. for 20 hours,obtaining a polymer of the following formula (P-0) in white powder solidform. The amount was 14.3 g and the yield was 88%.

Synthesis Example 2-2 Synthesis of Polymer (P-1)

10 g of Polymer (P-0) prepared in Synthesis Example 2-1, 4.0 g of PAG-1prepared in Synthesis Example 1-1, 50 g of methyl ethyl ketone, and 50 gof water were mixed and stirred at room temperature for 30 minutes. Theorganic layer was taken out from the mixture, 1.0 g of PAG-1 and 50 g ofwater were added to the organic layer, and the organic layer was takenout from the mixture. This reaction solution was washed with water 5times, and thereafter concentrated. Methyl isobutyl ketone was added tothe concentrate, followed by concentration again. Diisopropyl ether wasadded to the concentrate for recrystallization. The solid was collected,washed with diisopropyl ether, and vacuum dried at 50° C., obtaining 8.9g of the target polymer (P-1).

Synthesis Examples 2-3 to 2-14 Synthesis of Polymers (P-2) to (P-13)

A series of resins P-2 to P-13 as shown in Table 1 were prepared by thesame procedure as Synthesis Examples 2-1 and 2-2 while changing the typeand ratio of monomers. The units in Table 1 have the structure shown inTables 2 and 3. The structure of the sulfonium salt unit is as shownabove. In Table 1, the ratio of units is a molar ratio.

TABLE 1 Resin Unit 1 (ratio) Unit 2 (ratio) Unit 3 (ratio) Unit 4(ratio) P-1 PAG-2 (0.20) A-1 (0.30) B-1 (0.30) B-2 (0.20) P-2 PAG-2(0.30) A-1 (0.30) B-1 (0.20) B-2 (0.20) P-3 PAG-3 (0.20) A-1 (0.30) B-1(0.30) B-2 (0.20) P-4 PAG-3 (0.30) A-1 (0.30) B-1 (0.20) B-2 (0.20) P-5PAG-3 (0.20) A-1 (0.30) B-3 (0.30) B-2 (0.20) P-6 PAG-2 (0.20) A-1(0.30) B-3 (0.30) B-2 (0.20) P-7 PAG-3 (0.08) A-2 (0.55) B-4 (0.07) B-5(0.30) P-8 PAG-2 (0.08) A-2 (0.55) B-4 (0.07) B-5 (0.30) P-9 PAG-6(0.20) A-1 (0.30) B-1 (0.30) B-2 (0.20) P-10 PAG-5 (0.20) A-1 (0.30) B-3(0.30) B-2 (0.20) P-11 PAG-5 (0.08) A-2 (0.50) B-4 (0.22) B-5 (0.20)P-12 A-1 (0.30) B-1 (0.40) B-2 (0.30) — P-13 A-2 (0.50) B-4 (0.20) B-5(0.30) —

TABLE 2

A-1

A-2

TABLE 3

B-1

B-2

B-3

B-4

B-5

Comparative Synthesis Example 2-1

A nitrogen atmosphere flask was charged with 9.0 g of PAG-3, 4.9 g of3-ethyl-3-exo-tetracyclo[4.4.0.1^(2,5).1^(7,10)]-dodecanyl methacrylate,2.1 g of 4-hydroxyphenyl methacrylate, 4.0 g of4,8-dioxatricyclo[4.2.1.0^(3,7)]nonan-5-on-2-yl methacrylate, 1.4 g of2,2′-azobisisobutyronitrile, 17.5 g of methyl ethyl ketone, and 17.5 gof cyclohexanone to form a monomer solution. Another nitrogen atmosphereflask was charged with 5.8 g of methyl ethyl ketone and 5.8 g ofcyclohexanone, which were heated at 80° C. with stirring. The monomersolution was added dropwise to the solvent mixture over 4 hours. Afterthe completion of dropwise addition, the polymerization solution wasstirred for 2 hours while keeping it at 80° C. The polymerizationsolution was cooled to room temperature, after which it was addeddropwise to a mixture of 20.0 g of methyl ethyl ketone and 180 g ofhexane. The copolymer precipitate was collected by filtration. Thecopolymer was washed twice with a mixture of 37.0 g of methyl ethylketone and 83.1 g of hexane. It was vacuum dried at 50° C. for 20 hours,obtaining a white powder solid. On ¹H-NMR analysis, this white solid wasconfirmed distinct from the desired polymer (P-3) because copolymerizingunits of 3-ethyl-3-exo-tetracyclo[4.4.0.1^(2,5).1^(7,10)]dodecanylmethacrylate became methacrylic acid units as a result of elimination ofacid labile sites therefrom.

When synthesis was carried out by the same procedures as SynthesisExamples 2-1 and 2-2, the target polymer (P-3) could be synthesizedwithout decomposition.

Examples 1-1 to 1-8 and Comparative Examples 1-1 to 1-6

Resist compositions in solution form were prepared by mixing anddissolving the polymer (in Synthesis Example), PAG, amine quencher, andalkali-soluble surfactant (F-1) in a solvent according to theformulation shown in Table 4 and filtering through a Teflon® filterhaving a pore size of 0.2 μm. In all runs, the solvent contained 0.01 wt% of surfactant (F-2).

TABLE 4 Resin PAG Quencher Surfactant Solvent 1 Solvent 2 Resist (pbw)(pbw) (pbw) (pbw) (pbw) (pbw) Example 1-1 R-1  P-1  — Q-1 F-1 PGMEA CyHO(80) (1.4) (3.0) (576) (1,728) 1-2 R-2  P-2  — Q-1 F-1 PGMEA CyHO (80)(1.4) (3.0) (576) (1,728) 1-3 R-3  P-3  — Q-1 F-1 PGMEA CyHO (80) (1.4)(3.0) (576) (1,728) 1-4 R-4  P-4  — Q-1 F-1 PGMEA CyHO (80) (1.4) (3.0)(576) (1,728) 1-5 R-5  P-5  — Q-1 F-1 PGMEA CyHO (80) (1.4) (3.0) (576)(1,728) 1-6 R-6  P-6  — Q-1 F-1 PGMEA CyHO (80) (1.4) (3.0) (576)(1,728) 1-7 R-7  P-7  — Q-2 F-1 PGMEA CyHO (80) (1.4) (3.0) (576)(1,728) 1-8 R-8  P-8  — Q-2 F-1 PGMEA CyHO (80) (1.4) (3.0) (576)(1,728) Comparative Example 1-1 R-9  P-9  — Q-1 F-1 PGMEA CyHO (80)(1.4) (3.0) (576) (1,728) 1-2 R-10 P-10 — Q-1 F-1 PGMEA CyHO (80) (1.4)(3.0) (576) (1,728) 1-3 R-11 P-11 — Q-2 F-1 PGMEA CyHO (80) (1.4) (3.0)(576) (1,728) 1-4 R-12 P-12 PAG-1 Q-1 F-1 PGMEA CyHO (80) (5.1) (1.4)(3.0) (576) (1,728) 1-5 R-13 P-12 PAG-4 Q-1 F-1 PGMEA CyHO (80) (8.3)(1.4) (3.0) (576) (1,728) 1-6 R-14 P-13 PAG-4 Q-2 F-1 PGMEA CyHO (80)(8.3) (1.4) (3.0) (576) (1,728)

The photoacid generators PAG-1 and PAG-4 are as shown above. Thesolvent, amine quencher, alkali-soluble surfactant (F-1) and surfactant(F-2) used herein are identified below.

Organic Solvent

PGMEA: propylene glycol monomethyl ether acetate

CyHO: cyclohexanone

Quencher

Q-1: N-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)ethylmorpholine

Q-2: 2,6-diisopropylaniline

Surfactant F-1:poly(3,3,3-trifluoro-2-hydroxy-1,1-dimethyl-2-trifluoromethylpropylmethacrylate/1,1,1-trifluoro-2-hydroxy-6-methyl-2-trifluoromethylhept-4-ylmethacrylate) (described in JP 4900603)

F-2:3-methyl-3-(2,2,2-trifluoroethoxymethyl)oxetane/-tetrahydrofuran/2,2-dimethyl-1,3-propanediol copolymer (Omnova Solutions, Inc.)

EUV Exposure Test Examples 2-1 to 2-8 and Comparative Examples 2-1 to2-5

A silicon substrate of 8-inch diameter was coated with asilicon-containing SOG film of 35 nm thick (SHB-A940, Shin-Etsu ChemicalCo., Ltd.). The positive resist composition in Table 4 was coated on theSOG film and prebaked on a hot plate at 105° C. for 60 seconds to form aresist film of 50 nm thick. The wafer was exposed using an EUVmicrostepper (NA 0.3, quadrupole illumination). As a simulation of OOBlight irradiation, the wafer was exposed over the entire surface in adose of 1 mJ/cm² by means of an ArF excimer laser scanner and then overthe entire surface in a dose of 1 mJ/cm² by means of a KrF excimer laserscanner. This was followed by PEB on a hot plate at 95° C. for 60seconds and puddle development in a 2.38 wt % TMAH aqueous solution for30 seconds, yielding a positive pattern. The results are shown in Table5.

TABLE 5 Sensitivity Resist (mJ/cm²) Pattern profile Example 2-1 R-1 9.4rectangular 2-2 R-2 9.1 rectangular 2-3 R-3 9.5 rectangular 2-4 R-4 9.2rectangular 2-5 R-5 9.6 rectangular 2-6 R-6 9.6 rectangular 2-7 R-7 9.8rectangular 2-8 R-8 9.8 rectangular Comparative 2-1 R-9 9.9 tapered,film thickness loss Example 2-2 R-10 10.1 tapered, film thickness loss2-3 R-11 10.1 tapered, film thickness loss 2-4 R-13 10.1 tapered 2-5R-14 10.3 tapered

As is evident from Table 5, the resist compositions comprising sulfoniumsalt-containing polymers within the scope of the invention show a highsensitivity on EUV lithography. The rectangular profile of patternsuggests a low sensitivity to ArF and KrF exposures, that is, improvedresistance to OOB. In contrast, the resist compositions free of theinventive sulfonium salt-containing polymer (Comparative Examples 2-1 to2-5) are less resistant to OOB and sensitive to ArF and KrF exposuresand as a result, form patterns of tapered profile. Since there is astrong possibility that an actual EUV scanner used in practice is notequipped with an OOB-cutoff filter because of laser power shortage, theexperiment of pseudo-OOB irradiation by ArF and KrF lasers is asimulation of EUV scanner exposure.

EB Writing Test Examples 3-1 to 3-6 and Comparative Examples 3-1 to 3-4

Using a coater/developer system Clean Track Mark 5 (Tokyo ElectronLtd.), the resist composition in Table 4 was spin coated onto a siliconsubstrate (diameter 6 inches=150 mm, vapor primed withhexamethyldisilazane (HMDS)) and pre-baked on a hot plate at 110° C. for60 seconds to form a resist film of 100 nm thick. Using a system HL-800D(Hitachi Ltd.) at a HV voltage of 50 keV, the resist film was exposedimagewise to EB in a vacuum chamber.

Using Clean Track Mark 5, immediately after the imagewise exposure, theresist film was baked (PEB) on a hot plate at 95° C. for 60 seconds andpuddle developed in a 2.38 wt % TMAH aqueous solution for 30 seconds toform a positive pattern.

Sensitivity is the exposure dose that provides a 1:1 resolution of a100-nm line-and-space pattern. Resolution is a minimum size at theexposure dose. The 100-nm L/S pattern was measured for LWR under SEM.

Table 6 shows the sensitivity, resolution and LWR of resist compositionson EB lithography.

TABLE 6 Sensitivity Resolution LWR Resist (μC/cm²) (nm) (nm) Example 3-1R-1 25.0 75 5.7 3-2 R-2 21.8 75 5.5 3-3 R-3 24.8 75 5.4 3-4 R-4 22.0 755.1 3-5 R-5 25.2 75 5.6 3-6 R-6 25.1 75 5.8 Comparative 3-1 R-9 25.8 806.2 Example 3-2 R-10 26.4 80 6.9 3-3 R-12 26.8 85 7.5 3-4 R-13 27.0 857.8

As is evident from Table 6, the resist composition comprising asulfonium salt-containing polymer within the scope of the inventionshows a high resolution and a low LWR on EB lithography.

Japanese Patent Application No. 2012-269446 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A sulfonium salt having the general formula (1a):

wherein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, R^(1a) toR^(1m) are each independently hydrogen or a C₁-C₂₀ straight, branched orcyclic, monovalent hydrocarbon group which may be substituted with orseparated by a heteroatom, L is a single bond or a C₁-C₂₀ straight,branched or cyclic, divalent hydrocarbon group which may be substitutedwith or separated by a heteroatom, X is a C₁-C₅ divalent alkylene groupin which some or all hydrogen atoms may be substituted by fluorineatoms, and n is 0 or
 1. 2. A sulfonium salt having the general formula(1b):

wherein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, R^(1a) toR^(1m) are each independently hydrogen or a C₁-C₂₀ straight, branched orcyclic, monovalent hydrocarbon group which may be substituted with orseparated by a heteroatom, L¹ is a single bond or a C₁-C₂₀ straight,branched or cyclic, divalent hydrocarbon group which may be substitutedwith or separated by a heteroatom, A is hydrogen or trifluoromethyl, andn is 0 or 1, with the proviso that n is 0 when L¹ is a single bond.
 3. Apolymer comprising recurring units having the general formula (2a):

wherein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, R^(1a) toR^(1m) are each independently hydrogen or a C₁-C₂₀ straight, branched orcyclic, monovalent hydrocarbon group which may be substituted with orseparated by a heteroatom, L is a single bond or a C₁-C₂₀ straight,branched or cyclic, divalent hydrocarbon group which may be substitutedwith or separated by a heteroatom, X is a C₁-C₅ divalent alkylene groupin which some or all hydrogen atoms may be substituted by fluorineatoms, and n is 0 or
 1. 4. A polymer comprising recurring units havingthe general formula (2b):

wherein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, R^(1a) toR^(1m) are each independently hydrogen or a C₁-C₂₀ straight, branched orcyclic, monovalent hydrocarbon group which may be substituted with orseparated by a heteroatom, L¹ is a single bond or a C₁-C₂₀ straight,branched or cyclic, divalent hydrocarbon group which may be substitutedwith or separated by a heteroatom, A is hydrogen or trifluoromethyl, andn is 0 or 1, with the proviso that n is 0 when L¹ is a single bond. 5.The polymer of claim 3, further comprising recurring units having thegeneral formula (3) and/or (4):

wherein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, R² ishalogen or a C₁-C₁₀ alkyl group, L′ is a single bond or a C₁-C₁₀divalent organic group which may be substituted with oxygen, p is aninteger of 0 to 3, q is 1 or 2, N is an integer of 0 to 2, Z is a singlebond, phenylene, naphthylene or (backbone)-C(C═O)—O—Z′—, Z′ is a C₁-C₁₀straight, branched or cyclic alkylene group which may have a hydroxylradical, ether bond, ester bond or lactone ring, or phenylene ornaphthylene group, and XA is an acid labile group.
 6. The polymer ofclaim 5, further comprising recurring units having the general formula(5):

wherein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, and YL ishydrogen or a polar group having at least one structure selected fromthe group consisting of hydroxyl, cyano, carbonyl, carboxyl, ether bond,ester bond, sulfonic acid ester bond, carbonate bond, lactone ring,sultone ring and carboxylic anhydride.
 7. A resist compositioncomprising the polymer of claim 3 as a base resin.
 8. A resistcomposition comprising the polymer of claim 3 and a polymer free ofrecurring units having formulae (2a) and (2b) as a base resin.
 9. Theresist composition of claim 7, further comprising a basic compound andan organic solvent.
 10. The resist composition of claim 7, furthercomprising a non-polymeric acid generator.
 11. The resist composition ofclaim 7, further comprising a surfactant which is insoluble in water andsoluble in alkaline developer.
 12. A pattern forming process comprisingthe steps of applying the resist composition of claim 7 onto a substrateto form a coating, baking, exposing the coating to high-energyradiation, and developing the exposed coating in a developer.
 13. Theprocess of claim 12 wherein the exposure step is carried out byimmersion lithography using a liquid having a refractive index of atleast 1.0 between the resist coating and a projection lens.
 14. Theprocess of claim 13, further comprising the step of coating a protectivefilm on the resist coating prior to the exposure step, wherein immersionlithography is carried out while the liquid is held between theprotective film and the projection lens.
 15. The process of claim 12wherein the high-energy radiation is electron beam or soft X-ray havinga wavelength of 3 to 15 nm.
 16. A method for preparing the polymer ofclaim 3, comprising the step of reacting a polymer comprising recurringunits having the general formula (6a) with a sulfonium salt having thegeneral formula (7),

wherein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, R^(1a) toR^(1m) are each independently hydrogen or a C₁-C₂₀ straight, branched orcyclic, monovalent hydrocarbon group which may be substituted with orseparated by a heteroatom, L is a single bond or a C₁-C₂₀ straight,branched or cyclic, divalent hydrocarbon group which may be substitutedwith or separated by a heteroatom, X is a C₁-C₅ divalent alkylene groupin which some or all hydrogen atoms may be substituted by fluorineatoms, n is 0 or 1, Za⁺ is a lithium ion, sodium ion, potassium ion, orammonium cation of the general formula (8):(R³)₄N⁺  (8) wherein R³ is each independently hydrogen, a substituted orunsubstituted, C₁-C₁₀ straight, branched or cyclic alkyl, alkenyl oroxoalkyl group, or substituted or unsubstituted C₆-C₁₈ aryl, aralkyl oraryloxoalkyl group, or any two or more of R³ may bond together to form aring with N, and Xa⁻ is an anion.
 17. A method for preparing the polymerof claim 4, comprising the step of reacting a polymer comprisingrecurring units having the general formula (6b) with a sulfonium salthaving the general formula (7),

wherein R¹ is hydrogen, fluorine, methyl or trifluoromethyl, R^(1a) toR^(1m) are each independently hydrogen or a C₁-C₂₀ straight, branched orcyclic, monovalent hydrocarbon group which may be substituted with orseparated by a heteroatom, L¹ is a single bond or a C₁-C₂₀ straight,branched or cyclic, divalent hydrocarbon group which may be substitutedwith or separated by a heteroatom, A is hydrogen or trifluoromethyl, nis 0 or 1, Za⁺ is a lithium ion, sodium ion, potassium ion, or ammoniumcation of the general formula (8):(R³)₄N⁺  (8) wherein R³ is each independently hydrogen, a substituted orunsubstituted, C₁-C₁₀ straight, branched or cyclic alkyl, alkenyl oroxoalkyl group, or substituted or unsubstituted C₆-C₁₈ aryl, aralkyl oraryloxoalkyl group, or any two or more of R³ may bond together to form aring with N, and Xa⁻ is an anion.